feat(frontend): serialize/read helpers for plot config

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/tests/crossSubjectChart.test.js

@@ -4,6 +4,8 @@ import {
   setIdsHidden,
   groupVisibilityState,
   groupLines,
+  serializePlotConfig,
+  readPlotConfig,
 } from '../src/lib/crossSubjectChart';
 
 describe('sortPayloadByValueDesc', () => {
@@ -80,3 +82,69 @@ describe('groupLines', () => {
     expect(groupLines([{ id: '1' }])[0].group).toBe('—');
   });
 });
+
+describe('serializePlotConfig', () => {
+  const STATE = {
+    xField: '__days__', groupBy: 'g1', tickStep: 2, metric: 'Success',
+    hiddenSubjects: new Set(['b', 'a']), sidebarOpen: true,
+    counts: { height: 220, yMin: '0', yMax: '', xZoomMin: '', xZoomMax: '', xFieldOverride: 'f2' },
+    rate:   { height: 160, yMin: '', yMax: '', xZoomMin: '', xZoomMax: '', xFieldOverride: null },
+  };
+
+  it('produces a plain JSON object with hiddenSubjects as a sorted array', () => {
+    const out = serializePlotConfig(STATE);
+    expect(out.hiddenSubjects).toEqual(['a', 'b']);
+    expect(out.sidebarOpen).toBe(true);
+    expect(out.xField).toBe('__days__');
+    expect(out.metric).toBe('Success');
+    expect(out.counts).toEqual({ height: 220, yMin: '0', yMax: '', xZoomMin: '', xZoomMax: '', xFieldOverride: 'f2' });
+    expect(out.rate.xFieldOverride).toBeNull();
+  });
+
+  it('round-trips through readPlotConfig (values preserved)', () => {
+    const back = readPlotConfig(serializePlotConfig(STATE));
+    expect(back.xField).toBe('__days__');
+    expect(back.groupBy).toBe('g1');
+    expect(back.tickStep).toBe(2);
+    expect(back.hiddenSubjects).toEqual(['a', 'b']);
+    expect(back.sidebarOpen).toBe(true);
+    expect(back.counts.height).toBe(220);
+    expect(back.counts.xFieldOverride).toBe('f2');
+    expect(back.rate.height).toBe(160);
+    expect(back.rate.xFieldOverride).toBeNull();
+  });
+
+  it('returns an empty hiddenSubjects array when state.hiddenSubjects is null', () => {
+    expect(serializePlotConfig({ ...STATE, hiddenSubjects: null }).hiddenSubjects).toEqual([]);
+  });
+});
+
+describe('readPlotConfig', () => {
+  it('returns safe defaults for null', () => {
+    const c = readPlotConfig(null);
+    expect(c.hiddenSubjects).toEqual([]);
+    expect(c.sidebarOpen).toBe(false);
+    expect(c.xField).toBeUndefined();
+    expect(c.counts.yMin).toBe('');
+    expect(c.counts.xFieldOverride).toBeNull();
+  });
+
+  it('returns safe defaults for a non-object', () => {
+    expect(readPlotConfig([1, 2]).hiddenSubjects).toEqual([]);
+    expect(readPlotConfig('nope').sidebarOpen).toBe(false);
+  });
+
+  it('filters non-string hiddenSubjects and ignores wrong-typed fields', () => {
+    const c = readPlotConfig({ hiddenSubjects: ['a', 5, 'b'], tickStep: 'x', sidebarOpen: 'yes' });
+    expect(c.hiddenSubjects).toEqual(['a', 'b']);
+    expect(c.tickStep).toBeUndefined();
+    expect(c.sidebarOpen).toBe(false);
+  });
+
+  it('does not throw on malformed nested chart settings', () => {
+    expect(() => readPlotConfig({ counts: 'bad', rate: [] })).not.toThrow();
+    const c = readPlotConfig({ counts: 'bad' });
+    expect(c.counts.yMin).toBe('');
+    expect(c.counts.height).toBeUndefined();
+  });
+});