fix(detail): gap-honest year axis on the detail-page trend charts (Batch D)

Applies the compare screen's chart-truthfulness rules (spec §8.1) to the
school detail page's time-series charts.

PerformanceChart (Results Over Time, both phases):
- Fill every academic year between the first and last data point via the
  shared fillAcademicYears helper, so cancelled/unpublished years (2019/20,
  2020/21, and — for KS2 — 2021/22) render as real gaps instead of
  compressed time. Each series and the England overlay map onto this filled
  axis with null for missing years; spanGaps:false so school lines never
  bridge a gap.
- Replace the primary-only COVID note with a distinct, honest gap caption:
  KS2 names the cancelled tests plus the unpublished 2021/22 school-level
  year; KS4 names the unpublished 2019/20–2020/21 GCSE grading years.
- Mobile x-axis switches to autoSkip so the longer (gap-honest) axis stays
  readable; the broken line still marks a missing year even when its tick
  label is skipped.

AdmissionsTrendChart:
- Same gap-honest axis + spanGaps:false so a missing admissions year is a
  real gap, not compressed time.

Point 12 (definite canvas heights): desktop is already a definite 280px;
fix the secondary detail's mobile .chartContainer, which fixed the outer
box at 220px and double-constrained PerformanceChart's own 220px canvas +
chip strip (clipping the chips onto the plot) — now height:auto to match
the primary view.

SatsChart is out of scope (single-year per-subject CSS bars — no year axis,
no canvas height to constrain).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/components/PerformanceChart.tsx

@@ -16,6 +16,7 @@ import { ChartOptions, ChartDataset } from 'chart.js';
 import '@/lib/chartSetup';
 import type { SchoolResult } from '@/lib/types';
 import { formatAcademicYear } from '@/lib/utils';
+import { fillAcademicYears } from '@/lib/compareChartData';
 import { useIsMobile } from '@/hooks/useIsMobile';
 import { track } from '@/lib/analytics';
 import styles from './PerformanceChart.module.css';
@@ -35,8 +36,6 @@ interface PerformanceChartProps {
   nationalByYear?: NationalByYear[];
 }
 
-const COVID_YEARS = new Set([201920, 202021]);
-
 // Mobile chip definitions: which datasets render when each chip is active.
 // `series` keys reference the dataset labels so we can filter cleanly.
 type ChipId = 'expected' | 'higher' | 'progress' | 'attainment8' | 'em_pass' | 'progress8';
@@ -67,21 +66,33 @@ export function PerformanceChart({
   nationalByYear,
 }: PerformanceChartProps) {
   const sortedData = [...data].sort((a, b) => a.year - b.year);
-  const years = sortedData.map(d => formatAcademicYear(d.year));
+
+  // Gap-honest year axis: every academic year between the first and last data
+  // point appears, so cancelled/unpublished years (2019/20, 2020/21, and — for
+  // KS2 — 2021/22) render as real gaps rather than compressed time. School
+  // lines never bridge these gaps (spanGaps:false below).
+  const axisYears = fillAcademicYears(sortedData.map(d => d.year));
+  const byYear = new Map(sortedData.map(d => [d.year, d]));
+  const col = (key: keyof SchoolResult): (number | null)[] =>
+    axisYears.map(y => {
+      const v = byYear.get(y)?.[key];
+      return typeof v === 'number' ? v : null;
+    });
+  const years = axisYears.map(formatAcademicYear);
 
   const isMobile = useIsMobile();
 
-  // ── Build per-year national averages ─────────────────────────────────
-  const natRefRwm: (number | null)[] = sortedData.map(d => {
+  // ── Build per-year national averages (aligned to the filled axis) ────
+  const natRefRwm: (number | null)[] = axisYears.map(y => {
     if (nationalByYear) {
-      const match = nationalByYear.find(n => n.year === d.year);
+      const match = nationalByYear.find(n => n.year === y);
       return match?.primary?.rwm_expected_pct ?? null;
     }
     return nationalRwmAvg ?? null;
   });
-  const natRefAtt8: (number | null)[] = sortedData.map(d => {
+  const natRefAtt8: (number | null)[] = axisYears.map(y => {
     if (nationalByYear) {
-      const match = nationalByYear.find(n => n.year === d.year);
+      const match = nationalByYear.find(n => n.year === y);
       return match?.secondary?.attainment_8_score ?? null;
     }
     return nationalAtt8Avg ?? null;
@@ -107,10 +118,11 @@ export function PerformanceChart({
     return `${arrow} Reading, Writing & Maths peaked at ${bestPct}% (${formatAcademicYear(best.year)}), currently ${latestPct}%`;
   })();
 
-  const hasCovidGap = isSecondary
-    ? false
-    : COVID_YEARS.size > 0 &&
-      [...COVID_YEARS].some(y => !sortedData.find(d => d.year === y));
+  // A gap year is any filled axis year the school has no results row for —
+  // exactly the cancelled/unpublished years the fill introduced. Drives the
+  // honest note below (KS2 and KS4 have different gap stories).
+  const gapYears = axisYears.filter(y => !byYear.has(y));
+  const hasGap = gapYears.length > 0;
 
   // ── Datasets (full set; mobile filters them via the active chip) ─────
   const refLineStyle = {
@@ -120,13 +132,14 @@ export function PerformanceChart({
     borderDash: [6, 4] as number[],
     pointRadius: 0,
     tension: 0,
+    spanGaps: false as const,
     order: 10,
   };
 
   const allDatasets: ChartDataset<'line'>[] = isSecondary ? [
     {
       label: 'Attainment 8',
-      data: sortedData.map(d => d.attainment_8_score),
+      data: col('attainment_8_score'),
       borderColor: '#2d7d7d',
       backgroundColor: 'rgba(45,125,125,0.08)',
       borderWidth: 2.5,
@@ -137,7 +150,7 @@ export function PerformanceChart({
     },
     {
       label: 'English & Maths Grade 4+',
-      data: sortedData.map(d => d.english_maths_standard_pass_pct),
+      data: col('english_maths_standard_pass_pct'),
       borderColor: '#c9a227',
       backgroundColor: 'rgba(201,162,39,0.08)',
       borderWidth: 1.5,
@@ -147,7 +160,7 @@ export function PerformanceChart({
     },
     {
       label: 'Progress 8',
-      data: sortedData.map(d => d.progress_8_score),
+      data: col('progress_8_score'),
       borderColor: 'rgb(139,92,246)',
       backgroundColor: 'rgba(139,92,246,0.08)',
       borderWidth: 1.5,
@@ -165,7 +178,7 @@ export function PerformanceChart({
   ] : [
     {
       label: 'Reading, Writing & Maths expected %',
-      data: sortedData.map(d => d.rwm_expected_pct),
+      data: col('rwm_expected_pct'),
       borderColor: '#2d7d7d',
       backgroundColor: 'rgba(45,125,125,0.08)',
       borderWidth: 2.5,
@@ -176,7 +189,7 @@ export function PerformanceChart({
     },
     {
       label: 'Exceeding expected level',
-      data: sortedData.map(d => d.rwm_high_pct),
+      data: col('rwm_high_pct'),
       borderColor: '#c9a227',
       backgroundColor: 'rgba(201,162,39,0.08)',
       borderWidth: 1.5,
@@ -192,7 +205,7 @@ export function PerformanceChart({
     } as ChartDataset<'line'>] : []),
     {
       label: 'Reading progress',
-      data: sortedData.map(d => d.reading_progress),
+      data: col('reading_progress'),
       borderColor: 'rgb(59,130,246)',
       backgroundColor: 'rgba(59,130,246,0.08)',
       borderWidth: 1.5,
@@ -203,7 +216,7 @@ export function PerformanceChart({
     },
     {
       label: 'Writing progress',
-      data: sortedData.map(d => d.writing_progress),
+      data: col('writing_progress'),
       borderColor: 'rgb(139,92,246)',
       backgroundColor: 'rgba(139,92,246,0.08)',
       borderWidth: 1.5,
@@ -214,7 +227,7 @@ export function PerformanceChart({
     },
     {
       label: 'Maths progress',
-      data: sortedData.map(d => d.maths_progress),
+      data: col('maths_progress'),
       borderColor: 'rgb(236,72,153)',
       backgroundColor: 'rgba(236,72,153,0.08)',
       borderWidth: 1.5,
@@ -275,6 +288,8 @@ export function PerformanceChart({
     responsive: true,
     maintainAspectRatio: false,
     interaction: { mode: 'index', intersect: false },
+    // Never bridge missing years — cancelled/unpublished years are real gaps.
+    datasets: { line: { spanGaps: false } },
     plugins: {
       legend: {
         position: 'top',
@@ -326,6 +341,7 @@ export function PerformanceChart({
     responsive: true,
     maintainAspectRatio: false,
     interaction: { mode: 'index', intersect: false },
+    datasets: { line: { spanGaps: false } },
     plugins: {
       legend: { display: false },
       title: { display: false },
@@ -353,7 +369,10 @@ export function PerformanceChart({
       },
       x: {
         grid: { display: false },
-        ticks: { font: { size: 10 }, autoSkip: false },
+        // With the gap-honest axis (more year labels) autoSkip keeps the
+        // phone axis readable; the broken line still shows where a year is
+        // missing even when its tick label is skipped.
+        ticks: { font: { size: 10 }, autoSkip: true, maxTicksLimit: 5, maxRotation: 0 },
       },
     },
   };
@@ -416,9 +435,11 @@ export function PerformanceChart({
         </div>
       )}
 
-      {hasCovidGap && (
+      {hasGap && (
         <p className={styles.covidNote}>
-          * No data for 2019/20 or 2020/21 — national assessments were cancelled due to COVID-19.
+          {isSecondary
+            ? "School-level GCSE figures for 2019/20 and 2020/21 weren't published (COVID grading) where the line breaks — the England average is shown where available."
+            : "No national tests were held in 2019/20 and 2020/21 (COVID), and DfE didn't publish school-level figures for 2021/22 — the England average is shown for those years where available."}
         </p>
       )}