frontend: stacked-area NW history chart on the dashboard

First end-to-end view: ECharts stacked area of net worth by account
over the last 365 days, fed from GET /networth/history.

- NetWorthChart component with empty-state fallback
- Mocked ReactECharts in tests so they run without a DOM canvas
- Dashboard now: headline NW + history chart + per-account cards

Bundle grew to 467 KB gzipped — ECharts is heavy by design. Will
tree-shake via echarts/core imports once the chart surface stabilises.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/components/NetWorthChart.test.tsx

@@ -0,0 +1,39 @@
+import { render, screen } from '@testing-library/react';
+import { describe, it, expect, vi } from 'vitest';
+
+import { NetWorthChart } from './NetWorthChart';
+
+vi.mock('echarts-for-react', () => ({
+  default: ({ option }: { option: unknown }) => (
+    <div data-testid="chart">{JSON.stringify(option).length}</div>
+  ),
+}));
+
+describe('NetWorthChart', () => {
+  it('shows an empty-state when there are no points', () => {
+    render(<NetWorthChart history={{ points: [] }} />);
+    expect(screen.getByText('No history yet.')).toBeInTheDocument();
+  });
+
+  it('renders an ECharts option per account', () => {
+    render(
+      <NetWorthChart
+        history={{
+          points: [
+            {
+              snapshot_date: '2026-04-23',
+              total_gbp: '895000',
+              by_account: { ISA: '280000', Schwab: '615000' },
+            },
+            {
+              snapshot_date: '2026-04-25',
+              total_gbp: '940000',
+              by_account: { ISA: '300000', Schwab: '640000' },
+            },
+          ],
+        }}
+      />,
+    );
+    expect(screen.getByTestId('chart')).toBeInTheDocument();
+  });
+});

frontend/src/components/NetWorthChart.tsx

@@ -0,0 +1,78 @@
+/**
+ * Stacked area chart of net worth by account over time.
+ *
+ * Reads from `GET /networth/history` and groups by account name. Uses
+ * ECharts (sankey/fan/waterfall come from the same lib in later phases,
+ * keeping bundle simple).
+ */
+import { useMemo } from 'react';
+import ReactECharts from 'echarts-for-react';
+import type { EChartsOption } from 'echarts';
+
+import type { api } from '@/api/client';
+import { gbpCompact } from '@/lib/format';
+
+type History = Awaited<ReturnType<typeof api.networth.history>>;
+
+interface Props {
+  history: History;
+  height?: number;
+}
+
+export function NetWorthChart({ history, height = 360 }: Props) {
+  const option = useMemo<EChartsOption>(() => buildOption(history), [history]);
+
+  if (history.points.length === 0) {
+    return <p className="text-sm text-slate-500">No history yet.</p>;
+  }
+
+  return <ReactECharts option={option} style={{ height, width: '100%' }} notMerge lazyUpdate />;
+}
+
+function buildOption(history: History): EChartsOption {
+  const accountNames = collectAccountNames(history);
+
+  const seriesByAccount = accountNames.map((name) => ({
+    name,
+    type: 'line' as const,
+    stack: 'nw',
+    areaStyle: { opacity: 0.6 },
+    smooth: true,
+    showSymbol: false,
+    emphasis: { focus: 'series' as const },
+    data: history.points.map((p) => Number(p.by_account[name] ?? 0)),
+  }));
+
+  const dates = history.points.map((p) => p.snapshot_date);
+
+  return {
+    grid: { left: 60, right: 24, top: 24, bottom: 40 },
+    tooltip: {
+      trigger: 'axis',
+      valueFormatter: (v) => gbpCompact(typeof v === 'number' ? v : 0),
+    },
+    legend: {
+      bottom: 0,
+      type: 'scroll',
+    },
+    xAxis: {
+      type: 'category',
+      data: dates,
+      boundaryGap: false,
+      axisLabel: { hideOverlap: true },
+    },
+    yAxis: {
+      type: 'value',
+      axisLabel: { formatter: (v: number) => gbpCompact(v) },
+    },
+    series: seriesByAccount,
+  };
+}
+
+function collectAccountNames(history: History): string[] {
+  const names = new Set<string>();
+  for (const point of history.points) {
+    for (const name of Object.keys(point.by_account)) names.add(name);
+  }
+  return [...names].sort();
+}

frontend/src/pages/Dashboard.tsx

@@ -1,14 +1,23 @@
 /**
- * Dashboard — landing page. Wire up gets fleshed out in Phase 1a.
- * For now: confirm we can reach /networth and render today's NW.
+ * Dashboard — net worth at a glance.
+ *
+ * Three sections, top to bottom:
+ *   1. Headline NW total (latest snapshot)
+ *   2. Stacked-area history chart (per-account, /networth/history)
+ *   3. Per-account cards (latest values, /networth)
  */
 import { useQuery } from '@tanstack/react-query';
 
 import { api } from '@/api/client';
+import { NetWorthChart } from '@/components/NetWorthChart';
 import { gbp } from '@/lib/format';
 
 export function Dashboard() {
   const nw = useQuery({ queryKey: ['networth', 'current'], queryFn: api.networth.current });
+  const history = useQuery({
+    queryKey: ['networth', 'history', 365],
+    queryFn: () => api.networth.history(365),
+  });
 
   if (nw.isLoading) {
     return <p className="text-slate-500">Loading net worth…</p>;
@@ -45,10 +54,23 @@ export function Dashboard() {
         <h1 className="text-3xl font-semibold tracking-tight">Net worth</h1>
         <p className="text-sm text-slate-500">As of {data.snapshot_date}</p>
       </header>
+
       <div className="rounded-lg border border-slate-200 bg-white p-6">
         <div className="text-4xl font-semibold tabular-nums">{gbp(data.total_gbp)}</div>
         <p className="text-sm text-slate-500 mt-1">{data.accounts.length} accounts</p>
       </div>
+
+      <div className="rounded-lg border border-slate-200 bg-white p-6">
+        <h2 className="text-lg font-semibold mb-4">Last 12 months</h2>
+        {history.isLoading ? (
+          <p className="text-sm text-slate-500">Loading…</p>
+        ) : history.isError || !history.data ? (
+          <p className="text-sm text-slate-500">History unavailable.</p>
+        ) : (
+          <NetWorthChart history={history.data} />
+        )}
+      </div>
+
       <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-4">
         {data.accounts.map((a) => (
           <div