feat(compare): readable comparison chart on mobile

The compare chart squashed clustered schools into a thin band (y pinned
0-100) under an in-chart title + per-school legend that ate ~40% of a
300px card, leaving converging lines indistinguishable on phones.

- Auto-fit the y-axis to the data on all viewports (computeYBounds in
  lib/utils: padded + min-span for percentages, symmetric around 0 for
  progress, fitted for scores; negative pct-named trend metrics are not
  zero-clamped).
- Distinct point style per school (circle/triangle/rect/rectRot/star)
  as secondary encoding for convergence and colour-blindness.
- Mobile: drop in-chart title/legend/axis titles; add a chip row (colour
  dot + name) that doubles as tap-to-focus — highlights one school's
  line and dims the rest. Chart card 300px -> 340px, nearly all plot.
- Fix a latent colour mismatch: datasets were built from Object.entries
  whose integer-like URN keys enumerate in ascending numeric order,
  desyncing line colours from card colours; the chart now receives the
  ordered school list.
- Union years across schools instead of taking the first school's.
- Extract PerformanceChart's matchMedia pattern into hooks/useIsMobile.

Unit tests for metricKind/computeYBounds; e2e journey covers the mobile
chips and focus toggle.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

nextjs-app/components/ComparisonChart.tsx

@@ -1,47 +1,82 @@
 /**
  * ComparisonChart Component
- * Multi-school comparison chart using Chart.js
+ * Multi-school comparison chart using Chart.js.
+ *
+ * Desktop: built-in legend (point-style markers double as per-school shapes).
+ * Mobile (≤640px): the in-chart legend and axis titles are dropped in favour
+ *   of a chip row above the canvas; tapping a chip highlights that school's
+ *   line and dims the rest. The y-axis auto-fits the data on all viewports so
+ *   clustered schools stay distinguishable.
  */
 
 'use client';
 
+import { useEffect, useState } from 'react';
 import { Line } from 'react-chartjs-2';
-import { ChartOptions } from 'chart.js';
+import { ChartOptions, ChartDataset, PointStyle } from 'chart.js';
 import '@/lib/chartSetup';
 import type { ComparisonData } from '@/lib/types';
-import { CHART_COLORS, formatAcademicYear } from '@/lib/utils';
+import {
+  CHART_COLORS,
+  CHART_TEXT_COLORS,
+  computeYBounds,
+  formatAcademicYear,
+  metricKind,
+  rgbToRgba,
+} from '@/lib/utils';
+import { useIsMobile } from '@/hooks/useIsMobile';
+import { track } from '@/lib/analytics';
+import styles from './ComparisonChart.module.css';
 
 interface ComparisonChartProps {
   comparisonData: Record<string, ComparisonData>;
+  /** Ordered as displayed in the school cards, so colours match by index. */
+  schools: Array<{ urn: number; school_name: string }>;
   metric: string;
   metricLabel: string;
 }
 
-export function ComparisonChart({ comparisonData, metric, metricLabel }: ComparisonChartProps) {
-  // Get all schools and their data
-  const schools = Object.entries(comparisonData);
+// One shape per basket slot (MAX_SCHOOLS = 5) — secondary encoding so
+// converging lines stay tellable apart without relying on hue alone.
+const POINT_STYLES: PointStyle[] = ['circle', 'triangle', 'rect', 'rectRot', 'star'];
+
+export function ComparisonChart({ comparisonData, schools, metric, metricLabel }: ComparisonChartProps) {
+  const isMobile = useIsMobile();
+  const [focusedUrn, setFocusedUrn] = useState<number | null>(null);
+
+  // A focused school that leaves the basket must not linger.
+  const urnKey = schools.map((s) => s.urn).join(',');
+  useEffect(() => {
+    setFocusedUrn(null);
+  }, [urnKey]);
 
   if (schools.length === 0) {
     return <div>No data available</div>;
   }
 
-  // Get years from first school (assuming all schools have same years)
-  const years = schools[0][1].yearly_data.map((d) => d.year).sort((a, b) => a - b);
+  // Union of years across all schools — coverage differs between them.
+  const years = [
+    ...new Set(schools.flatMap((s) => comparisonData[String(s.urn)]?.yearly_data.map((d) => d.year) ?? [])),
+  ].sort((a, b) => a - b);
 
-  // Create datasets for each school
-  const datasets = schools.map(([urn, data], index) => {
-    const schoolInfo = data.school_info;
+  const datasets: ChartDataset<'line'>[] = schools.map((school, index) => {
+    const data = comparisonData[String(school.urn)];
     const color = CHART_COLORS[index % CHART_COLORS.length];
+    const dimmed = focusedUrn !== null && focusedUrn !== school.urn;
 
     return {
-      label: schoolInfo.school_name,
+      label: school.school_name,
       data: years.map((year) => {
-        const yearData = data.yearly_data.find((d) => d.year === year);
+        const yearData = data?.yearly_data.find((d) => d.year === year);
         if (!yearData) return null;
         return yearData[metric as keyof typeof yearData] as number | null;
       }),
-      borderColor: color,
-      backgroundColor: color.replace('rgb', 'rgba').replace(')', ', 0.1)'),
+      borderColor: dimmed ? rgbToRgba(color, 0.2) : color,
+      backgroundColor: dimmed ? 'transparent' : rgbToRgba(color, 0.1),
+      borderWidth: focusedUrn === school.urn ? 3 : dimmed ? 1.5 : 2,
+      pointStyle: POINT_STYLES[index % POINT_STYLES.length],
+      pointRadius: dimmed ? 2 : isMobile ? 3 : 4,
+      pointHoverRadius: isMobile ? 5 : 6,
       tension: 0.3,
       spanGaps: true,
     };
@@ -52,9 +87,11 @@ export function ComparisonChart({ comparisonData, metric, metricLabel }: Compari
     datasets,
   };
 
-  // Determine if metric is a progress score or percentage
-  const isProgressScore = metric.includes('progress');
-  const isPercentage = metric.includes('pct') || metric.includes('rate');
+  const kind = metricKind(metric);
+  const yBounds = computeYBounds(
+    datasets.flatMap((ds) => ds.data as Array<number | null>),
+    kind,
+  );
 
   const options: ChartOptions<'line'> = {
     responsive: true,
@@ -65,6 +102,7 @@ export function ComparisonChart({ comparisonData, metric, metricLabel }: Compari
     },
     plugins: {
       legend: {
+        display: !isMobile,
         position: 'top' as const,
         labels: {
           usePointStyle: true,
@@ -74,26 +112,22 @@ export function ComparisonChart({ comparisonData, metric, metricLabel }: Compari
           },
         },
       },
+      // No in-chart title: the section heading and metric selector above the
+      // chart already state the metric.
       title: {
-        display: true,
-        text: `${metricLabel} - Comparison`,
-        font: {
-          size: 16,
-          weight: 'bold',
-        },
-        padding: {
-          bottom: 20,
-        },
+        display: false,
       },
       tooltip: {
         backgroundColor: 'rgba(0, 0, 0, 0.8)',
-        padding: 12,
+        padding: isMobile ? 10 : 12,
         titleFont: {
-          size: 14,
+          size: isMobile ? 12 : 14,
         },
         bodyFont: {
-          size: 13,
+          size: isMobile ? 11 : 13,
         },
+        usePointStyle: true,
+        itemSort: (a, b) => (b.parsed.y ?? -Infinity) - (a.parsed.y ?? -Infinity),
         callbacks: {
           label: function (context) {
             let label = context.dataset.label || '';
@@ -101,13 +135,7 @@ export function ComparisonChart({ comparisonData, metric, metricLabel }: Compari
               label += ': ';
             }
             if (context.parsed.y !== null) {
-              if (isProgressScore) {
-                label += context.parsed.y.toFixed(1);
-              } else if (isPercentage) {
-                label += context.parsed.y.toFixed(1) + '%';
-              } else {
-                label += context.parsed.y.toFixed(1);
-              }
+              label += context.parsed.y.toFixed(1) + (kind === 'percentage' ? '%' : '');
             } else {
               label += 'N/A';
             }
@@ -121,17 +149,18 @@ export function ComparisonChart({ comparisonData, metric, metricLabel }: Compari
         type: 'linear' as const,
         display: true,
         title: {
-          display: true,
-          text: isPercentage ? 'Percentage (%)' : isProgressScore ? 'Progress Score' : 'Value',
+          display: !isMobile,
+          text: kind === 'percentage' ? 'Percentage (%)' : kind === 'progress' ? 'Progress Score' : 'Value',
           font: {
             size: 12,
             weight: 'bold',
           },
         },
-        ...(isPercentage && {
-          min: 0,
-          max: 100,
-        }),
+        ...yBounds,
+        ticks: {
+          font: { size: isMobile ? 10 : 12 },
+          ...(isMobile && { maxTicksLimit: 5 }),
+        },
         grid: {
           color: 'rgba(0, 0, 0, 0.05)',
         },
@@ -141,16 +170,58 @@ export function ComparisonChart({ comparisonData, metric, metricLabel }: Compari
           display: false,
         },
         title: {
-          display: true,
+          display: !isMobile,
           text: 'Year',
           font: {
             size: 12,
             weight: 'bold',
           },
         },
+        ticks: {
+          font: { size: isMobile ? 10 : 12 },
+          ...(isMobile && { maxRotation: 0, autoSkip: true, maxTicksLimit: 4 }),
+        },
       },
     },
   };
 
-  return <Line data={chartData} options={options} />;
+  const toggleFocus = (urn: number) => {
+    const next = focusedUrn === urn ? null : urn;
+    setFocusedUrn(next);
+    if (next !== null) track('compare_focus_school', { urn: next });
+  };
+
+  return (
+    <div className={styles.wrapper}>
+      {/* Mobile legend + focus control; a single series needs no legend. */}
+      {schools.length > 1 && (
+        <div className={styles.chips} role="group" aria-label="Highlight a school on the chart">
+          {schools.map((school, index) => (
+            <button
+              key={school.urn}
+              type="button"
+              className={styles.chip}
+              aria-pressed={focusedUrn === school.urn}
+              onClick={() => toggleFocus(school.urn)}
+            >
+              <span
+                className={styles.chipDot}
+                style={{ background: CHART_COLORS[index % CHART_COLORS.length] }}
+                aria-hidden="true"
+              />
+              <span
+                className={styles.chipName}
+                style={{ color: CHART_TEXT_COLORS[index % CHART_TEXT_COLORS.length] }}
+              >
+                {school.school_name}
+              </span>
+            </button>
+          ))}
+        </div>
+      )}
+      <div className={styles.canvasBox}>
+        <Line data={chartData} options={options} aria-label={`${metricLabel} comparison chart`} />
+      </div>
+    </div>
+  );
 }