feat(export): toCSV emits context line + subject group cell

frontend/src/lib/dataExport.js

@@ -206,12 +206,35 @@ function escapeCsvCell(value) {
   return /[",\n]/.test(s) ? `"${s.replace(/"/g, '""')}"` : s;
 }
 
-// Serialize a matrix to an RFC-4180-ish CSV string (LF line endings).
-// First column is the date; remaining columns follow matrix.columns order.
+// Serialize a matrix to an RFC-4180-ish CSV string (LF line endings):
+//   <pinned label>,<pinned value>
+//   (blank line)
+//   [Group,<group per subject column>]    -- only when groupAxis === 'col'
+//   <corner>[,Group?],<column headers...>
+//   <row label>[,group?],<values...>
 export function toCSV(matrix) {
-  const header = ['Date', ...matrix.columns.map((c) => c.header)].map(escapeCsvCell).join(',');
-  const lines = matrix.rows.map((r) => [r.date, ...r.values].map(escapeCsvCell).join(','));
-  return [header, ...lines].join('\n');
+  const lines = [];
+  lines.push([matrix.context.label, matrix.context.value].map(escapeCsvCell).join(','));
+  lines.push('');
+
+  if (matrix.groupAxis === 'col') {
+    const groupRow = ['Group', ...matrix.columns.map((c) => c.group ?? '—')];
+    lines.push(groupRow.map(escapeCsvCell).join(','));
+  }
+
+  const header = matrix.groupAxis === 'row'
+    ? ['Group', matrix.corner, ...matrix.columns.map((c) => c.label)]
+    : [matrix.corner, ...matrix.columns.map((c) => c.label)];
+  lines.push(header.map(escapeCsvCell).join(','));
+
+  for (const r of matrix.rows) {
+    const lead = matrix.groupAxis === 'row'
+      ? [r.member.group ?? '—', r.member.label]
+      : [r.member.label];
+    lines.push([...lead, ...r.values].map(escapeCsvCell).join(','));
+  }
+
+  return lines.join('\n');
 }
 
 // Build a download filename from the experiment title and parameter label.

frontend/tests/dataExport.test.js

@@ -141,20 +141,58 @@ describe('buildMatrix', () => {
 import { toCSV, csvFilename } from '../src/lib/dataExport';
 
 describe('toCSV', () => {
-  it('writes a header row of Date + column headers, then data rows', () => {
+  it('emits context line, blank line, header, data (no group)', () => {
     const matrix = {
-      columns: [{ id: 'a1', header: 'Alpha' }, { id: 'a2', header: 'Beta' }],
-      rows: [{ date: '2026-07-01', values: [5, ''] }, { date: '2026-07-02', values: ['', 9] }],
-    };
-    expect(toCSV(matrix)).toBe('Date,Alpha,Beta\n2026-07-01,5,\n2026-07-02,,9');
-  });
-  it('escapes commas, quotes, and newlines per RFC 4180', () => {
-    const matrix = {
-      columns: [{ id: 'a1', header: 'Note, field' }],
-      rows: [{ date: '2026-07-01', values: ['he said "hi"'] }, { date: '2026-07-02', values: ['line1\nline2'] }],
+      corner: 'Date',
+      context: { label: 'Parameter', value: 'Total attempts' },
+      groupAxis: null,
+      columns: [{ id: 'a1', label: 'Alpha', group: null }, { id: 'a2', label: 'Beta', group: null }],
+      rows: [{ member: { label: '2026-07-01' }, values: [5, ''] }, { member: { label: '2026-07-02' }, values: ['', 9] }],
     };
     expect(toCSV(matrix)).toBe(
-      'Date,"Note, field"\n2026-07-01,"he said ""hi"""\n2026-07-02,"line1\nline2"',
+      'Parameter,Total attempts\n\nDate,Alpha,Beta\n2026-07-01,5,\n2026-07-02,,9',
+    );
+  });
+
+  it('adds a Group header row when subjects are columns', () => {
+    const matrix = {
+      corner: 'Date',
+      context: { label: 'Parameter', value: 'Total attempts' },
+      groupAxis: 'col',
+      columns: [{ id: 'a1', label: 'Alpha', group: 'Control' }, { id: 'a2', label: 'Beta', group: 'Drug' }],
+      rows: [{ member: { label: '2026-07-01' }, values: [5, 9] }],
+    };
+    expect(toCSV(matrix)).toBe(
+      'Parameter,Total attempts\n\nGroup,Control,Drug\nDate,Alpha,Beta\n2026-07-01,5,9',
+    );
+  });
+
+  it('adds a leading Group column when subjects are rows', () => {
+    const matrix = {
+      corner: 'Subject',
+      context: { label: 'Date', value: '2026-07-01' },
+      groupAxis: 'row',
+      columns: [{ id: 'p1', label: 'Total attempts', group: 'metrics' }],
+      rows: [
+        { member: { label: 'Alpha', group: 'Control' }, values: [5] },
+        { member: { label: 'Beta', group: 'Drug' }, values: [9] },
+      ],
+    };
+    expect(toCSV(matrix)).toBe(
+      'Date,2026-07-01\n\nGroup,Subject,Total attempts\nControl,Alpha,5\nDrug,Beta,9',
+    );
+  });
+
+  it('escapes commas, quotes, and newlines per RFC 4180', () => {
+    const matrix = {
+      corner: 'Date',
+      context: { label: 'Parameter', value: 'Note, field' },
+      groupAxis: null,
+      columns: [{ id: 'a1', label: 'he said "hi"', group: null }],
+      rows: [{ member: { label: '2026-07-01' }, values: ['line1\nline2'] }],
+    };
+    expect(toCSV(matrix)).toBe(
+      'Parameter,"Note, field"\n\nDate,"he said ""hi"""\n2026-07-01,"line1\nline2"',
     );
   });
 });