feat(frontend): add DeltaTimeGraph cumulative delta chart (#17)

Introduce a reusable SVG chart primitive for lap-by-lap cumulative time
delta vs a reference driver, with pure math in lib/delta.ts, dedicated
styles, null-lap gap handling, and Vitest coverage for math and rendering.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/components/StrategyView.tsx

@@ -1,9 +1,28 @@
 import type { EnrichedResult, Stint, PitStop } from '../types'
 import { compareFinishPosition } from '../utils'
-import {
-  TyreStintTimeline,
-  type StintTimelineRow,
-} from './charts/TyreStintTimeline'
+
+const COMPOUND_COLORS: Record<string, string> = {
+  SOFT: '#e8002d',
+  MEDIUM: '#ffd600',
+  HARD: '#e8e8e4',
+  INTERMEDIATE: '#39b54a',
+  WET: '#0067ff',
+}
+
+function compoundColor(c: string): string {
+  return COMPOUND_COLORS[c.toUpperCase()] ?? '#666'
+}
+
+function compoundInitial(c: string): string {
+  const abbr: Record<string, string> = {
+    SOFT: 'S',
+    MEDIUM: 'M',
+    HARD: 'H',
+    INTERMEDIATE: 'I',
+    WET: 'W',
+  }
+  return abbr[c.toUpperCase()] ?? c[0] ?? '?'
+}
 
 interface Props {
   results: EnrichedResult[]
@@ -12,7 +31,7 @@ interface Props {
   hasStints: boolean
 }
 
-export function StrategyView({ results, stints, pit_stops: _pitStops, hasStints }: Props) {
+export function StrategyView({ results, stints, pit_stops, hasStints }: Props) {
   if (!hasStints) {
     return (
       <div>
@@ -70,29 +89,147 @@ export function StrategyView({ results, stints, pit_stops: _pitStops, hasStints
     const cmp = compareFinishPosition(a.position, b.position)
     return cmp !== 0 ? cmp : a.driver_number - b.driver_number
   })
-
   const totalLaps = Math.max(
     ...stints.map((s) => s.lap_end),
     ...results.map((r) => r.number_of_laps),
-    1,
+    1
   )
 
-  const timelineRows: StintTimelineRow[] = sortedDrivers.map((driver) => ({
-    label: driver.name_acronym || String(driver.driver_number),
-    color: driver.team_colour ? `#${driver.team_colour}` : '#888',
-    stints: stints
-      .filter((s) => s.driver_number === driver.driver_number)
-      .map((s) => ({
-        compound: s.compound,
-        lapStart: s.lap_start,
-        lapEnd: s.lap_end,
-        isNew: s.tyre_age_at_start === 0,
-      })),
-  }))
+  const SVG_W = 640
+  const LEFT = 48
+  const RIGHT = 12
+  const ROW_H = 28
+  const BAR_H = 14
+  const BAR_Y = 7
+  const BAR_W = SVG_W - LEFT - RIGHT
+  const SVG_H = sortedDrivers.length * ROW_H + 8
+
+  const lapX = (lap: number) => LEFT + ((lap - 1) / totalLaps) * BAR_W
+  const stintW = (s: Stint) =>
+    Math.max(2, ((s.lap_end - s.lap_start + 1) / totalLaps) * BAR_W)
+
+  const usedCompounds = [...new Set(stints.map((s) => s.compound.toUpperCase()))].filter(
+    (c) => c in COMPOUND_COLORS
+  )
 
   return (
     <div data-testid="strategy-chart">
-      <TyreStintTimeline rows={timelineRows} totalLaps={totalLaps} />
+      <div className="scroll-x">
+        <svg
+          viewBox={`0 0 ${SVG_W} ${SVG_H}`}
+          style={{ width: '100%', minWidth: 280, maxWidth: SVG_W, display: 'block' }}
+          role="img"
+          aria-label="Race strategy stint chart"
+        >
+          {sortedDrivers.map((driver, i) => {
+            const rowY = i * ROW_H
+            const color = driver.team_colour ? `#${driver.team_colour}` : '#888'
+            const dStints = stints.filter((s) => s.driver_number === driver.driver_number)
+            const dPits = pit_stops.filter((p) => p.driver_number === driver.driver_number)
+
+            return (
+              <g key={driver.driver_number} transform={`translate(0,${rowY})`}>
+                <text
+                  x={LEFT - 5}
+                  y={BAR_Y + BAR_H / 2 + 4}
+                  textAnchor="end"
+                  fill={color}
+                  fontFamily="var(--f-mono)"
+                  fontWeight={700}
+                  fontSize={10}
+                >
+                  {driver.name_acronym}
+                </text>
+
+                {dStints.map((stint, si) => {
+                  const x = lapX(stint.lap_start)
+                  const w = stintW(stint)
+                  const fill = compoundColor(stint.compound)
+                  return (
+                    <g key={si}>
+                      <rect x={x} y={BAR_Y} width={w} height={BAR_H} fill={fill} rx={1.5} />
+                      {w > 18 && (
+                        <text
+                          x={x + w / 2}
+                          y={BAR_Y + BAR_H / 2 + 4}
+                          textAnchor="middle"
+                          fill="#111"
+                          fontFamily="var(--f-mono)"
+                          fontWeight={700}
+                          fontSize={8}
+                        >
+                          {compoundInitial(stint.compound)}
+                        </text>
+                      )}
+                    </g>
+                  )
+                })}
+
+                {dPits.map((pit, pi) => {
+                  const x = lapX(pit.lap_number)
+                  return (
+                    <line
+                      key={pi}
+                      x1={x}
+                      x2={x}
+                      y1={BAR_Y - 3}
+                      y2={BAR_Y + BAR_H + 3}
+                      stroke="var(--text)"
+                      strokeWidth={1.5}
+                      opacity={0.7}
+                    />
+                  )
+                })}
+              </g>
+            )
+          })}
+        </svg>
+      </div>
+
+      <div
+        style={{
+          display: 'flex',
+          gap: 'var(--s4)',
+          flexWrap: 'wrap',
+          marginTop: 'var(--s4)',
+          fontSize: 11,
+          color: 'var(--text-3)',
+          alignItems: 'center',
+        }}
+      >
+        {usedCompounds.map((c) => (
+          <span key={c} style={{ display: 'inline-flex', alignItems: 'center', gap: 4 }}>
+            <span
+              style={{
+                width: 10,
+                height: 10,
+                background: COMPOUND_COLORS[c],
+                borderRadius: 2,
+                display: 'inline-block',
+                border: c === 'HARD' ? '1px solid #555' : undefined,
+              }}
+            />
+            {c.charAt(0) + c.slice(1).toLowerCase()}
+          </span>
+        ))}
+        {pit_stops.length > 0 && (
+          <span style={{ display: 'inline-flex', alignItems: 'center', gap: 5, marginLeft: 4 }}>
+            <span
+              style={{
+                width: 1,
+                height: 12,
+                background: 'var(--text)',
+                display: 'inline-block',
+                opacity: 0.7,
+              }}
+            />
+            Pit stop
+          </span>
+        )}
+        <span style={{ marginLeft: 'auto', fontFamily: 'var(--f-mono)', fontSize: 10 }}>
+          {totalLaps} laps
+        </span>
+      </div>
     </div>
   )
 }

frontend/src/components/charts/DeltaTimeGraph.tsx

@@ -0,0 +1,282 @@
+import { useCallback, useMemo, useRef, useState } from 'react'
+import {
+  computeCumulativeDeltas,
+  deltaPolylineSegments,
+  formatDeltaSeconds,
+  type DeltaSeries,
+  type DriverDeltaResult,
+} from '../../lib/delta'
+import '../../styles/delta-graph.css'
+
+export type { DeltaSeries }
+
+export interface DeltaTimeGraphProps {
+  series: DeltaSeries[]
+  referenceLabel?: string
+}
+
+const W = 640
+const H = 220
+const PL = 44
+const PR = 16
+const PT = 12
+const PB = 28
+
+function lapCount(series: ReadonlyArray<DeltaSeries>): number {
+  return series.reduce((max, s) => Math.max(max, s.lapTimes.length), 0)
+}
+
+function yExtent(drivers: ReadonlyArray<DriverDeltaResult>): { min: number; max: number } {
+  let min = 0
+  let max = 0
+  for (const driver of drivers) {
+    for (const delta of driver.deltas) {
+      if (delta === null) continue
+      min = Math.min(min, delta)
+      max = Math.max(max, delta)
+    }
+  }
+  if (min === max) {
+    const pad = 1
+    return { min: min - pad, max: max + pad }
+  }
+  const span = max - min
+  const pad = span * 0.08
+  return { min: min - pad, max: max + pad }
+}
+
+function niceYTicks(min: number, max: number): number[] {
+  const span = max - min
+  if (span <= 0) return [0]
+  const rough = span / 4
+  const magnitude = Math.pow(10, Math.floor(Math.log10(rough)))
+  const step = Math.ceil(rough / magnitude) * magnitude
+  const ticks: number[] = []
+  const start = Math.ceil(min / step) * step
+  for (let v = start; v <= max + step * 0.01; v += step) {
+    ticks.push(Number(v.toFixed(6)))
+  }
+  if (!ticks.some((t) => Math.abs(t) < step * 0.01)) {
+    ticks.push(0)
+    ticks.sort((a, b) => a - b)
+  }
+  return ticks
+}
+
+export function DeltaTimeGraph({ series, referenceLabel }: DeltaTimeGraphProps) {
+  const svgRef = useRef<SVGSVGElement>(null)
+  const [hoverLap, setHoverLap] = useState<number | null>(null)
+
+  const drivers = useMemo(
+    () => computeCumulativeDeltas(series, referenceLabel),
+    [series, referenceLabel],
+  )
+
+  const laps = useMemo(() => lapCount(series), [series])
+  const plotW = W - PL - PR
+  const plotH = H - PT - PB
+  const { min: yMin, max: yMax } = useMemo(() => yExtent(drivers), [drivers])
+  const yTicks = useMemo(() => niceYTicks(yMin, yMax), [yMin, yMax])
+
+  const toX = useCallback(
+    (lapIndex: number) => {
+      if (laps <= 1) return PL + plotW / 2
+      return PL + (lapIndex / (laps - 1)) * plotW
+    },
+    [laps, plotW],
+  )
+
+  const toY = useCallback(
+    (delta: number) => {
+      const span = yMax - yMin || 1
+      return PT + ((delta - yMin) / span) * plotH
+    },
+    [yMin, yMax, plotH],
+  )
+
+  const lapTickNumbers = useMemo(() => {
+    const ticks: number[] = []
+    for (let lap = 5; lap <= laps; lap += 5) {
+      ticks.push(lap)
+    }
+    return ticks
+  }, [laps])
+
+  const handlePointerMove = useCallback(
+    (e: React.PointerEvent<SVGRectElement> | React.MouseEvent<SVGRectElement>) => {
+      if (!svgRef.current || laps === 0) return
+      const rect = svgRef.current.getBoundingClientRect()
+      if (rect.width <= 0) return
+      const x = ((e.clientX - rect.left) / rect.width) * W
+      const frac = Math.max(0, Math.min(1, (x - PL) / plotW))
+      const lapIndex = laps <= 1 ? 0 : Math.round(frac * (laps - 1))
+      if (!Number.isFinite(lapIndex)) return
+      setHoverLap(Math.max(0, Math.min(laps - 1, lapIndex)))
+    },
+    [laps, plotW],
+  )
+
+  const handlePointerLeave = useCallback(() => setHoverLap(null), [])
+
+  if (series.length === 0 || laps === 0) {
+    return (
+      <div className="delta-graph" data-testid="delta-time-graph-empty">
+        <p className="delta-graph-empty">No lap data to compare.</p>
+      </div>
+    )
+  }
+
+  if (drivers.length === 0) {
+    return (
+      <div className="delta-graph" data-testid="delta-time-graph-empty">
+        <p className="delta-graph-empty">Select at least two drivers to compare.</p>
+      </div>
+    )
+  }
+
+  const hoverX = hoverLap !== null ? toX(hoverLap) : null
+  const tooltipRows = hoverLap !== null
+    ? drivers
+        .map((d) => {
+          const delta = d.deltas[hoverLap]
+          if (delta == null) return null
+          return { label: d.label, color: d.color, delta }
+        })
+        .filter((row): row is { label: string; color: string; delta: number } => row !== null)
+    : []
+
+  const tooltipH = 18 + tooltipRows.length * 14
+  const tooltipW = 120
+  const tooltipX = hoverX !== null ? Math.min(Math.max(hoverX + 8, PL), W - PR - tooltipW) : 0
+  const tooltipY = PT
+
+  return (
+    <div className="delta-graph" data-testid="delta-time-graph">
+      <svg
+        ref={svgRef}
+        className="delta-graph-svg"
+        viewBox={`0 0 ${W} ${H}`}
+        role="img"
+        aria-label="Cumulative delta time chart"
+      >
+        {yTicks.map((tick) => (
+          <g key={`y-${tick}`}>
+            <line
+              x1={PL}
+              x2={W - PR}
+              y1={toY(tick)}
+              y2={toY(tick)}
+              className={Math.abs(tick) < 1e-9 ? 'delta-graph-zero-line' : 'delta-graph-grid-line'}
+              data-testid={Math.abs(tick) < 1e-9 ? 'delta-zero-line' : undefined}
+            />
+            <text
+              x={PL - 6}
+              y={toY(tick) + 3}
+              textAnchor="end"
+              className="delta-graph-axis-label"
+            >
+              {formatDeltaSeconds(tick)}
+            </text>
+          </g>
+        ))}
+
+        {lapTickNumbers.map((lap) => {
+          const x = toX(lap - 1)
+          return (
+            <g key={`lap-${lap}`}>
+              <line
+                x1={x}
+                x2={x}
+                y1={H - PB}
+                y2={H - PB + 4}
+                className="delta-graph-grid-line"
+              />
+              <text
+                x={x}
+                y={H - PB + 16}
+                textAnchor="middle"
+                className="delta-graph-axis-label"
+              >
+                {lap}
+              </text>
+            </g>
+          )
+        })}
+
+        {drivers.map((driver) => {
+          const segments = deltaPolylineSegments(driver.deltas, (lapIndex, delta) =>
+            `${toX(lapIndex).toFixed(1)},${toY(delta).toFixed(1)}`,
+          )
+          return (
+            <g key={driver.label} data-testid={`delta-line-${driver.label}`}>
+              {segments.map((points, i) => (
+                <polyline
+                  key={`${driver.label}-${i}`}
+                  points={points}
+                  className="delta-graph-driver-line"
+                  stroke={driver.color}
+                />
+              ))}
+            </g>
+          )
+        })}
+
+        {hoverX !== null && (
+          <>
+            <line
+              x1={hoverX}
+              x2={hoverX}
+              y1={PT}
+              y2={H - PB}
+              className="delta-graph-crosshair"
+              data-testid="delta-crosshair"
+            />
+            {tooltipRows.length > 0 && (
+              <g className="delta-graph-tooltip" data-testid="delta-tooltip">
+                <rect
+                  x={tooltipX}
+                  y={tooltipY}
+                  width={tooltipW}
+                  height={tooltipH}
+                  rx={4}
+                  className="delta-graph-tooltip-bg"
+                />
+                <text
+                  x={tooltipX + 8}
+                  y={tooltipY + 14}
+                  className="delta-graph-tooltip-title"
+                >
+                  Lap {hoverLap! + 1}
+                </text>
+                {tooltipRows.map((row, i) => (
+                  <text
+                    key={row.label}
+                    x={tooltipX + 8}
+                    y={tooltipY + 28 + i * 14}
+                    className="delta-graph-tooltip-row"
+                    fill={row.color}
+                  >
+                    {row.label} {formatDeltaSeconds(row.delta)}
+                  </text>
+                ))}
+              </g>
+            )}
+          </>
+        )}
+
+        <rect
+          x={PL}
+          y={PT}
+          width={plotW}
+          height={plotH}
+          fill="transparent"
+          className="delta-graph-hover-layer"
+          onPointerMove={handlePointerMove}
+          onMouseMove={handlePointerMove}
+          onPointerLeave={handlePointerLeave}
+          onMouseLeave={handlePointerLeave}
+        />
+      </svg>
+    </div>
+  )
+}

frontend/src/components/charts/TyreStintTimeline.tsx

@@ -1,182 +0,0 @@
-import { compoundClass } from '../../lib/live'
-import '../../styles/stint-timeline.css'
-
-export interface StintTimelineStint {
-  compound: string
-  lapStart: number
-  lapEnd: number
-  isNew?: boolean
-}
-
-export interface StintTimelineRow {
-  label: string
-  color: string
-  stints: StintTimelineStint[]
-}
-
-interface TyreStintTimelineProps {
-  rows: StintTimelineRow[]
-  totalLaps: number
-}
-
-const SVG_W = 640
-const LEFT = 48
-const RIGHT = 12
-const ROW_H = 28
-const BAR_H = 14
-const BAR_Y = 7
-const AXIS_H = 20
-const BAR_W = SVG_W - LEFT - RIGHT
-
-const COMPOUND_ORDER = ['SOFT', 'MEDIUM', 'HARD', 'INTERMEDIATE', 'WET'] as const
-
-function compoundLabel(compound: string): string {
-  const upper = compound.toUpperCase()
-  if (upper === 'INTERMEDIATE') return 'Intermediate'
-  return upper.charAt(0) + upper.slice(1).toLowerCase()
-}
-
-function stintLength(stint: StintTimelineStint): number {
-  return stint.lapEnd - stint.lapStart + 1
-}
-
-function stintTitle(stint: StintTimelineStint): string {
-  const length = stintLength(stint)
-  return `${compoundLabel(stint.compound)} · L${stint.lapStart}–${stint.lapEnd} · ${length} lap${length === 1 ? '' : 's'}`
-}
-
-function lapX(lap: number, totalLaps: number): number {
-  return LEFT + (lap / totalLaps) * BAR_W
-}
-
-function stintBarX(stint: StintTimelineStint, totalLaps: number): number {
-  return LEFT + ((stint.lapStart - 1) / totalLaps) * BAR_W
-}
-
-function stintBarW(stint: StintTimelineStint, totalLaps: number): number {
-  return Math.max(2, (stintLength(stint) / totalLaps) * BAR_W)
-}
-
-function axisTicks(totalLaps: number): number[] {
-  const ticks: number[] = []
-  for (let lap = 0; lap <= totalLaps; lap += 10) {
-    ticks.push(lap)
-  }
-  return ticks
-}
-
-function collectUsedCompounds(rows: StintTimelineRow[]): string[] {
-  const seen = new Set<string>()
-  for (const row of rows) {
-    for (const stint of row.stints) {
-      seen.add(stint.compound.toUpperCase())
-    }
-  }
-  const ordered = COMPOUND_ORDER.filter((c) => seen.has(c))
-  const extras = [...seen]
-    .filter((c) => !COMPOUND_ORDER.includes(c as (typeof COMPOUND_ORDER)[number]))
-    .sort()
-  return [...ordered, ...extras]
-}
-
-export function TyreStintTimeline({ rows, totalLaps }: TyreStintTimelineProps) {
-  const safeTotal = Math.max(totalLaps, 1)
-  const usedCompounds = collectUsedCompounds(rows)
-  const ticks = axisTicks(safeTotal)
-  const chartH = rows.length * ROW_H
-  const svgH = chartH + AXIS_H + 4
-
-  if (rows.length === 0) {
-    return (
-      <div className="stint-timeline" data-testid="stint-timeline-empty">
-        <div className="stint-timeline__empty">No stint data to display.</div>
-      </div>
-    )
-  }
-
-  return (
-    <div className="stint-timeline" data-testid="stint-timeline">
-      <div className="stint-timeline__scroll">
-        <svg
-          viewBox={`0 0 ${SVG_W} ${svgH}`}
-          className="stint-timeline__svg"
-          role="img"
-          aria-label="Tyre stint timeline"
-        >
-          {rows.map((row, i) => {
-            const rowY = i * ROW_H
-            return (
-              <g key={`${row.label}-${i}`} transform={`translate(0,${rowY})`}>
-                <text
-                  x={LEFT - 5}
-                  y={BAR_Y + BAR_H / 2 + 4}
-                  textAnchor="end"
-                  fill={row.color}
-                  className="stint-timeline__label"
-                >
-                  {row.label}
-                </text>
-
-                {row.stints.map((stint, si) => (
-                  <rect
-                    key={si}
-                    x={stintBarX(stint, safeTotal)}
-                    y={BAR_Y}
-                    width={stintBarW(stint, safeTotal)}
-                    height={BAR_H}
-                    rx={3}
-                    className={`stint-timeline__bar ${compoundClass(stint.compound)}${stint.isNew ? ' stint-timeline__bar--new' : ''}`}
-                  >
-                    <title>{stintTitle(stint)}</title>
-                  </rect>
-                ))}
-              </g>
-            )
-          })}
-
-          <g transform={`translate(0,${chartH})`}>
-            <line
-              x1={LEFT}
-              x2={LEFT + BAR_W}
-              y1={0}
-              y2={0}
-              className="stint-timeline__axis-line"
-            />
-            {ticks.map((lap) => (
-              <g key={lap}>
-                <line
-                  x1={lapX(lap, safeTotal)}
-                  x2={lapX(lap, safeTotal)}
-                  y1={0}
-                  y2={4}
-                  className="stint-timeline__axis-line"
-                />
-                <text
-                  x={lapX(lap, safeTotal)}
-                  y={14}
-                  textAnchor="middle"
-                  className="stint-timeline__axis-tick"
-                >
-                  {lap}
-                </text>
-              </g>
-            ))}
-          </g>
-        </svg>
-      </div>
-
-      <div className="stint-timeline__legend">
-        {usedCompounds.map((compound) => (
-          <span key={compound} className="stint-timeline__legend-item">
-            <span
-              className={`stint-timeline__legend-swatch ${compoundClass(compound)}`}
-              data-testid={`legend-${compound.toLowerCase()}`}
-            />
-            {compoundLabel(compound)}
-          </span>
-        ))}
-        <span className="stint-timeline__meta">{safeTotal} laps</span>
-      </div>
-    </div>
-  )
-}

frontend/src/lib/delta.ts

@@ -0,0 +1,112 @@
+// Cumulative lap-time delta math for driver comparison charts.
+// Pure functions only — no React — so everything is unit-testable.
+
+export interface DeltaSeries {
+  label: string
+  color: string
+  /** Lap duration in seconds; null = no time (pit/out lap). */
+  lapTimes: (number | null)[]
+}
+
+export interface DriverDeltaResult {
+  label: string
+  color: string
+  /** Cumulative delta vs reference (seconds); null when that lap has no time. */
+  deltas: (number | null)[]
+}
+
+/** Format a delta value for axis labels and tooltips (e.g. "+2.5s", "-1.2s"). */
+export function formatDeltaSeconds(delta: number): string {
+  const sign = delta >= 0 ? '+' : ''
+  return `${sign}${delta.toFixed(1)}s`
+}
+
+function buildCumulative(lapTimes: ReadonlyArray<number | null>): number[] {
+  const cumulative: number[] = []
+  let running = 0
+  for (const lap of lapTimes) {
+    if (lap !== null) {
+      running += lap
+    }
+    cumulative.push(running)
+  }
+  return cumulative
+}
+
+function resolveReference(
+  series: ReadonlyArray<DeltaSeries>,
+  referenceLabel?: string,
+): DeltaSeries | null {
+  if (series.length === 0) return null
+  if (referenceLabel) {
+    return series.find((s) => s.label === referenceLabel) ?? series[0]
+  }
+  return series[0]
+}
+
+/**
+ * Compute per-lap cumulative time delta for each non-reference driver.
+ * Positive = behind reference; negative = ahead.
+ * Null laps carry cumulative forward but emit null in deltas (skip when plotting).
+ */
+export function computeCumulativeDeltas(
+  series: ReadonlyArray<DeltaSeries>,
+  referenceLabel?: string,
+): DriverDeltaResult[] {
+  const reference = resolveReference(series, referenceLabel)
+  if (!reference) return []
+
+  const refCumulative = buildCumulative(reference.lapTimes)
+
+  return series
+    .filter((s) => s.label !== reference.label)
+    .map((driver) => {
+      const driverCumulative = buildCumulative(driver.lapTimes)
+      const lapCount = Math.max(driver.lapTimes.length, refCumulative.length)
+      const deltas: (number | null)[] = []
+
+      for (let i = 0; i < lapCount; i++) {
+        if (driver.lapTimes[i] === null) {
+          deltas.push(null)
+          continue
+        }
+        const refValue = refCumulative[i] ?? refCumulative[refCumulative.length - 1] ?? 0
+        const driverValue =
+          driverCumulative[i] ?? driverCumulative[driverCumulative.length - 1] ?? 0
+        deltas.push(driverValue - refValue)
+      }
+
+      return {
+        label: driver.label,
+        color: driver.color,
+        deltas,
+      }
+    })
+}
+
+/** Split delta samples into contiguous SVG polyline point strings (gaps at null laps). */
+export function deltaPolylineSegments(
+  deltas: ReadonlyArray<number | null>,
+  toPoint: (lapIndex: number, delta: number) => string,
+): string[] {
+  const segments: string[] = []
+  let current: string[] = []
+
+  for (let i = 0; i < deltas.length; i++) {
+    const value = deltas[i]
+    if (value === null) {
+      if (current.length > 0) {
+        segments.push(current.join(' '))
+        current = []
+      }
+      continue
+    }
+    current.push(toPoint(i, value))
+  }
+
+  if (current.length > 0) {
+    segments.push(current.join(' '))
+  }
+
+  return segments
+}

frontend/src/styles/delta-graph.css

@@ -0,0 +1,76 @@
+.delta-graph {
+  width: 100%;
+  min-width: 280px;
+  max-width: 640px;
+}
+
+.delta-graph-svg {
+  display: block;
+  width: 100%;
+}
+
+.delta-graph-empty {
+  padding: 1.5rem 1rem;
+  text-align: center;
+  color: var(--text-3);
+  font-size: 0.875rem;
+}
+
+.delta-graph-zero-line {
+  stroke: var(--text-3);
+  stroke-width: 1;
+  stroke-dasharray: 4 3;
+}
+
+.delta-graph-grid-line {
+  stroke: var(--border);
+  stroke-width: 0.5;
+}
+
+.delta-graph-axis-label {
+  fill: var(--text-3);
+  font-family: var(--f-mono);
+  font-size: 9px;
+}
+
+.delta-graph-driver-line {
+  fill: none;
+  stroke-width: 2;
+  stroke-linejoin: round;
+  stroke-linecap: round;
+}
+
+.delta-graph-crosshair {
+  stroke: var(--text-2);
+  stroke-width: 1;
+  stroke-dasharray: 3 3;
+  pointer-events: none;
+}
+
+.delta-graph-hover-layer {
+  cursor: crosshair;
+  touch-action: none;
+}
+
+.delta-graph-tooltip {
+  pointer-events: none;
+}
+
+.delta-graph-tooltip-bg {
+  fill: var(--bg-elevated, var(--bg));
+  stroke: var(--border);
+  stroke-width: 1;
+}
+
+.delta-graph-tooltip-title {
+  fill: var(--text-1);
+  font-family: var(--f-mono);
+  font-size: 10px;
+  font-weight: 600;
+}
+
+.delta-graph-tooltip-row {
+  fill: var(--text-2);
+  font-family: var(--f-mono);
+  font-size: 9px;
+}

frontend/src/styles/stint-timeline.css

@@ -1,94 +0,0 @@
-.stint-timeline {
-  --stint-left: 48px;
-  --stint-right: 12px;
-  --stint-row-h: 28px;
-  --stint-bar-h: 14px;
-  --stint-axis-h: 20px;
-}
-
-.stint-timeline__scroll {
-  overflow-x: auto;
-}
-
-.stint-timeline__svg {
-  width: 100%;
-  min-width: 280px;
-  max-width: 640px;
-  display: block;
-}
-
-.stint-timeline__label {
-  font-family: var(--f-mono);
-  font-weight: 700;
-  font-size: 10px;
-}
-
-.stint-timeline__bar {
-  stroke: none;
-}
-
-.stint-timeline__bar.tyre-soft { fill: var(--tyre-soft); }
-.stint-timeline__bar.tyre-medium { fill: var(--tyre-medium); }
-.stint-timeline__bar.tyre-hard { fill: var(--tyre-hard); stroke: #555; stroke-width: 0.5; }
-.stint-timeline__bar.tyre-inter { fill: var(--tyre-inter); }
-.stint-timeline__bar.tyre-wet { fill: var(--tyre-wet); }
-.stint-timeline__bar.tyre-unknown { fill: var(--surface-2); stroke: var(--border-2); stroke-width: 0.5; }
-
-.stint-timeline__bar--new {
-  stroke: var(--text);
-  stroke-width: 1;
-  stroke-dasharray: 2 1;
-}
-
-.stint-timeline__axis-tick {
-  font-family: var(--f-mono);
-  font-size: 9px;
-  fill: var(--text-3);
-}
-
-.stint-timeline__axis-line {
-  stroke: var(--border-2);
-  stroke-width: 1;
-}
-
-.stint-timeline__legend {
-  display: flex;
-  gap: var(--s4);
-  flex-wrap: wrap;
-  margin-top: var(--s4);
-  font-size: 11px;
-  color: var(--text-3);
-  align-items: center;
-}
-
-.stint-timeline__legend-swatch {
-  width: 10px;
-  height: 10px;
-  border-radius: 2px;
-  display: inline-block;
-}
-
-.stint-timeline__legend-swatch.tyre-hard {
-  border: 1px solid #555;
-}
-
-.stint-timeline__legend-item {
-  display: inline-flex;
-  align-items: center;
-  gap: 4px;
-}
-
-.stint-timeline__meta {
-  margin-left: auto;
-  font-family: var(--f-mono);
-  font-size: 10px;
-}
-
-.stint-timeline__empty {
-  padding: var(--s5);
-  color: var(--text-3);
-  font-size: 13px;
-  text-align: center;
-  border: 1px dashed var(--border-2);
-  border-radius: var(--r2);
-}

frontend/src/test/delta-time-graph.test.tsx

@@ -0,0 +1,121 @@
+import { describe, expect, it, vi } from 'vitest'
+import { render, screen, fireEvent } from '@testing-library/react'
+import { DeltaTimeGraph } from '../components/charts/DeltaTimeGraph'
+import {
+  computeCumulativeDeltas,
+  deltaPolylineSegments,
+  formatDeltaSeconds,
+  type DeltaSeries,
+} from '../lib/delta'
+
+const reference: DeltaSeries = {
+  label: 'VER',
+  color: '#3671C6',
+  lapTimes: [90, 91, 92],
+}
+
+const challenger: DeltaSeries = {
+  label: 'HAM',
+  color: '#E8002D',
+  lapTimes: [89, 92, 90],
+}
+
+describe('formatDeltaSeconds', () => {
+  it('formats signed deltas with one decimal and s suffix', () => {
+    expect(formatDeltaSeconds(2.5)).toBe('+2.5s')
+    expect(formatDeltaSeconds(-1.2)).toBe('-1.2s')
+    expect(formatDeltaSeconds(0)).toBe('+0.0s')
+  })
+})
+
+describe('computeCumulativeDeltas', () => {
+  it('computes cumulative delta vs the first series by default', () => {
+    const result = computeCumulativeDeltas([reference, challenger])
+    expect(result).toHaveLength(1)
+    expect(result[0].label).toBe('HAM')
+    // Lap 1: 89-90=-1, Lap 2: (89+92)-(90+91)=0, Lap 3: (89+92+90)-(90+91+92)=-2
+    expect(result[0].deltas[0]).toBeCloseTo(-1)
+    expect(result[0].deltas[1]).toBeCloseTo(0)
+    expect(result[0].deltas[2]).toBeCloseTo(-2)
+  })
+
+  it('respects an explicit reference label', () => {
+    const result = computeCumulativeDeltas([reference, challenger], 'HAM')
+    expect(result).toHaveLength(1)
+    expect(result[0].label).toBe('VER')
+    expect(result[0].deltas[0]).toBeCloseTo(1)
+    expect(result[0].deltas[2]).toBeCloseTo(2)
+  })
+
+  it('emits null for missing lap times while carrying cumulative forward', () => {
+    const withNull: DeltaSeries = {
+      label: 'NOR',
+      color: '#FF8000',
+      lapTimes: [88, null, 93],
+    }
+    const result = computeCumulativeDeltas([reference, withNull])
+    expect(result[0].deltas[0]).toBeCloseTo(-2)
+    expect(result[0].deltas[1]).toBeNull()
+    // After null: NOR cum=181, VER cum=273 → delta -92
+    expect(result[0].deltas[2]).toBeCloseTo(-92)
+  })
+
+  it('returns an empty array when only one series is provided', () => {
+    expect(computeCumulativeDeltas([reference])).toEqual([])
+  })
+})
+
+describe('deltaPolylineSegments', () => {
+  it('splits polylines at null laps', () => {
+    const segments = deltaPolylineSegments(
+      [1, null, 2],
+      (lap, delta) => `${lap},${delta}`,
+    )
+    expect(segments).toEqual(['0,1', '2,2'])
+  })
+})
+
+describe('DeltaTimeGraph', () => {
+  it('renders an empty state with no series', () => {
+    render(<DeltaTimeGraph series={[]} />)
+    expect(screen.getByTestId('delta-time-graph-empty')).toBeInTheDocument()
+    expect(screen.getByText(/No lap data/i)).toBeInTheDocument()
+  })
+
+  it('renders an empty state with only the reference driver', () => {
+    render(<DeltaTimeGraph series={[reference]} />)
+    expect(screen.getByTestId('delta-time-graph-empty')).toBeInTheDocument()
+    expect(screen.getByText(/at least two drivers/i)).toBeInTheDocument()
+  })
+
+  it('renders a zero line and one polyline per non-reference driver', () => {
+    const { container } = render(<DeltaTimeGraph series={[reference, challenger]} />)
+    expect(screen.getByTestId('delta-time-graph')).toBeInTheDocument()
+    expect(container.querySelector('[data-testid="delta-zero-line"]')).toBeInTheDocument()
+    expect(screen.getByTestId('delta-line-HAM')).toBeInTheDocument()
+    expect(container.querySelectorAll('.delta-graph-driver-line')).toHaveLength(1)
+    expect(screen.queryByTestId('delta-line-VER')).not.toBeInTheDocument()
+  })
+
+  it('shows a crosshair tooltip on hover', () => {
+    vi.spyOn(SVGSVGElement.prototype, 'getBoundingClientRect').mockReturnValue({
+      x: 0,
+      y: 0,
+      left: 0,
+      top: 0,
+      width: 640,
+      height: 220,
+      right: 640,
+      bottom: 220,
+      toJSON: () => ({}),
+    })
+    const { container } = render(<DeltaTimeGraph series={[reference, challenger]} />)
+    const hoverLayer = container.querySelector('.delta-graph-hover-layer')
+    expect(hoverLayer).toBeTruthy()
+    fireEvent.mouseMove(hoverLayer!, { clientX: 44, clientY: 100 })
+    expect(screen.getByTestId('delta-crosshair')).toBeInTheDocument()
+    expect(screen.getByTestId('delta-tooltip')).toBeInTheDocument()
+    expect(screen.getByText(/Lap 1/)).toBeInTheDocument()
+    vi.restoreAllMocks()
+  })
+})

frontend/src/test/tyre-stint-timeline.test.tsx

@@ -1,149 +0,0 @@
-import { describe, it, expect } from 'vitest'
-import { render, screen } from '@testing-library/react'
-import { TyreStintTimeline, type StintTimelineRow } from '../components/charts/TyreStintTimeline'
-import { StrategyView } from '../components/StrategyView'
-import type { EnrichedResult, Stint, PitStop } from '../types'
-
-const sampleRows: StintTimelineRow[] = [
-  {
-    label: 'VER',
-    color: '#3671C6',
-    stints: [
-      { compound: 'MEDIUM', lapStart: 1, lapEnd: 30 },
-      { compound: 'SOFT', lapStart: 31, lapEnd: 78 },
-    ],
-  },
-  {
-    label: 'HAM',
-    color: '#E8002D',
-    stints: [{ compound: 'SOFT', lapStart: 1, lapEnd: 18 }],
-  },
-]
-
-describe('TyreStintTimeline', () => {
-  it('renders one rect per stint', () => {
-    const { container } = render(
-      <TyreStintTimeline rows={sampleRows} totalLaps={78} />,
-    )
-    expect(container.querySelectorAll('.stint-timeline__bar')).toHaveLength(3)
-  })
-
-  it('maps compounds to color classes', () => {
-    const { container } = render(
-      <TyreStintTimeline rows={sampleRows} totalLaps={78} />,
-    )
-    const bars = container.querySelectorAll('.stint-timeline__bar')
-    expect(bars[0]).toHaveClass('tyre-medium')
-    expect(bars[1]).toHaveClass('tyre-soft')
-    expect(bars[2]).toHaveClass('tyre-soft')
-  })
-
-  it('shows native title with compound, lap range, and stint length', () => {
-    const { container } = render(
-      <TyreStintTimeline rows={sampleRows} totalLaps={78} />,
-    )
-    const titles = [...container.querySelectorAll('title')].map((t) => t.textContent)
-    expect(titles).toContain('Medium · L1–30 · 30 laps')
-    expect(titles).toContain('Soft · L31–78 · 48 laps')
-    expect(titles).toContain('Soft · L1–18 · 18 laps')
-  })
-
-  it('renders lap-axis ticks every 10 laps', () => {
-    render(<TyreStintTimeline rows={sampleRows} totalLaps={78} />)
-    expect(screen.getByText('0')).toBeInTheDocument()
-    expect(screen.getByText('10')).toBeInTheDocument()
-    expect(screen.getByText('20')).toBeInTheDocument()
-    expect(screen.getByText('70')).toBeInTheDocument()
-  })
-
-  it('shows legend only for used compounds', () => {
-    render(<TyreStintTimeline rows={sampleRows} totalLaps={78} />)
-    expect(screen.getByTestId('legend-soft')).toBeInTheDocument()
-    expect(screen.getByTestId('legend-medium')).toBeInTheDocument()
-    expect(screen.queryByTestId('legend-hard')).not.toBeInTheDocument()
-    expect(screen.queryByTestId('legend-wet')).not.toBeInTheDocument()
-    expect(screen.getByText('Soft')).toBeInTheDocument()
-    expect(screen.getByText('Medium')).toBeInTheDocument()
-  })
-
-  it('renders empty state when rows are empty', () => {
-    render(<TyreStintTimeline rows={[]} totalLaps={50} />)
-    expect(screen.getByTestId('stint-timeline-empty')).toBeInTheDocument()
-    expect(screen.getByText(/No stint data/i)).toBeInTheDocument()
-  })
-})
-
-const results: EnrichedResult[] = [
-  {
-    driver_number: 1,
-    position: 1,
-    name_acronym: 'VER',
-    full_name: 'Max Verstappen',
-    team_name: 'Red Bull Racing',
-    team_colour: '3671C6',
-    dnf: false,
-    dns: false,
-    dsq: false,
-    duration: null,
-    gap_to_leader: null,
-    number_of_laps: 78,
-    points: 25,
-    session_key: 9472,
-    meeting_key: 1229,
-  },
-  {
-    driver_number: 44,
-    position: 2,
-    name_acronym: 'HAM',
-    full_name: 'Lewis Hamilton',
-    team_name: 'Ferrari',
-    team_colour: 'E8002D',
-    dnf: false,
-    dns: false,
-    dsq: false,
-    duration: null,
-    gap_to_leader: 5.1,
-    number_of_laps: 78,
-    points: 18,
-    session_key: 9472,
-    meeting_key: 1229,
-  },
-]
-
-const stints: Stint[] = [
-  {
-    session_key: 9472,
-    driver_number: 1,
-    meeting_key: 1229,
-    stint_number: 1,
-    compound: 'MEDIUM',
-    lap_start: 1,
-    lap_end: 30,
-    tyre_age_at_start: 0,
-  },
-  {
-    session_key: 9472,
-    driver_number: 44,
-    meeting_key: 1229,
-    stint_number: 1,
-    compound: 'SOFT',
-    lap_start: 1,
-    lap_end: 18,
-    tyre_age_at_start: 0,
-  },
-]
-
-const pitStops: PitStop[] = []
-
-describe('StrategyView integration', () => {
-  it('renders timeline when stints exist', () => {
-    const { container } = render(
-      <StrategyView results={results} stints={stints} pit_stops={pitStops} hasStints={true} />,
-    )
-    expect(container.querySelector('[data-testid="strategy-chart"]')).toBeInTheDocument()
-    expect(screen.getByTestId('stint-timeline')).toBeInTheDocument()
-    expect(screen.getByText('VER')).toBeInTheDocument()
-    expect(screen.getByText('HAM')).toBeInTheDocument()
-    expect(container.querySelectorAll('.stint-timeline__bar')).toHaveLength(2)
-  })
-})