feat(analytics): stacked category chart, savings rate line, expandable rows

- Replace grouped cashflow BarChart with ComposedChart: expense categories
  as colour-coded stacked bars + amber savings-rate % line on right Y-axis
- Add category colour legend below chart (matches stacked bars)
- Horizontal category bar chart now uses per-category colours
- Breakdown table: click any category row to expand/collapse individual
  transactions; each transaction has an inline category dropdown that
  calls PATCH /api/transactions/:id → transaction_overrides, then
  invalidates analytics query so totals update immediately

src/app/budget/page.tsx

@@ -2,17 +2,19 @@
 
 import { useState } from "react";
 import {
+  ComposedChart,
   BarChart,
   Bar,
+  Line,
   XAxis,
   YAxis,
   Tooltip,
   ResponsiveContainer,
   Cell,
-  Legend,
 } from "recharts";
-import { useMonthlyAnalytics } from "@/lib/hooks";
+import { useQueryClient } from "@tanstack/react-query";
-import { formatCategory } from "@/lib/categories";
+import { useMonthlyAnalytics, useTransactions, useUpdateTransaction } from "@/lib/hooks";
+import { formatCategory, CATEGORIES } from "@/lib/categories";
 
 function currentMonthStr(): string {
   const now = new Date();
@@ -64,8 +66,165 @@ const TOOLTIP_STYLE = {
   fontSize: 12,
 };
 
+const CATEGORY_COLORS: Record<string, string> = {
+  groceries: "#22c55e",
+  dining: "#f97316",
+  transport: "#06b6d4",
+  fuel: "#eab308",
+  shopping: "#ec4899",
+  utilities: "#8b5cf6",
+  entertainment: "#f43f5e",
+  travel: "#0ea5e9",
+  health: "#10b981",
+  insurance: "#64748b",
+  subscriptions: "#a78bfa",
+  cash_advance: "#dc2626",
+  government: "#78716c",
+  education: "#3b82f6",
+  rent: "#d97706",
+  transfers: "#6b7280",
+  income: "#34d399",
+  investment: "#818cf8",
+  personal_care: "#fb7185",
+  pets: "#86efac",
+  gifts: "#fcd34d",
+  charity: "#a3e635",
+  other: "#71717a",
+};
+
+interface TooltipPayload {
+  name: string;
+  value: number;
+  fill?: string;
+  stroke?: string;
+}
+
+function CashflowTooltip({
+  active,
+  payload,
+  label,
+}: {
+  active?: boolean;
+  payload?: TooltipPayload[];
+  label?: string;
+}) {
+  if (!active || !payload) return null;
+  const items = payload.filter((p) => p.value && Math.abs(p.value) > 0.01);
+  if (items.length === 0) return null;
+
+  return (
+    <div style={TOOLTIP_STYLE} className="p-2.5 text-xs min-w-36">
+      <p className="text-zinc-400 mb-2 font-medium">{label}</p>
+      {items.map((item) => {
+        const name = item.name;
+        let label = name;
+        let valueStr = fmtExact(Number(item.value));
+        const color = item.fill || item.stroke || "#71717a";
+
+        if (name === "income") label = "Income";
+        else if (name === "savingsRate") {
+          label = "Savings Rate";
+          valueStr = `${Number(item.value).toFixed(1)}%`;
+        } else if (name.startsWith("cat_")) {
+          label = formatCategory(name.slice(4));
+        }
+
+        return (
+          <div key={name} className="flex items-center gap-2 mb-0.5">
+            <span className="w-2 h-2 rounded-sm shrink-0" style={{ background: color }} />
+            <span className="text-zinc-400">{label}:</span>
+            <span className="text-zinc-100 tabular-nums ml-auto pl-2">{valueStr}</span>
+          </div>
+        );
+      })}
+    </div>
+  );
+}
+
+function CategoryPanel({
+  category,
+  selectedMonth,
+}: {
+  category: string;
+  selectedMonth: string;
+}) {
+  const qc = useQueryClient();
+  const updateTx = useUpdateTransaction();
+
+  const from = `${selectedMonth}-01`;
+  const [year, month] = selectedMonth.split("-").map(Number);
+  const nextDate = new Date(year, month, 1);
+  const to = `${nextDate.getFullYear()}-${String(nextDate.getMonth() + 1).padStart(2, "0")}-01`;
+
+  const { data, isLoading } = useTransactions({ category, from, to, limit: 200 });
+  const txns = data?.data || [];
+
+  const handleCategoryChange = (id: number, newCategory: string) => {
+    updateTx.mutate(
+      { id, category: newCategory },
+      {
+        onSuccess: () => {
+          qc.invalidateQueries({ queryKey: ["analytics"] });
+        },
+      }
+    );
+  };
+
+  return (
+    <tr>
+      <td colSpan={4} className="px-0 pb-2 bg-zinc-950/60 border-b border-zinc-800">
+        {isLoading ? (
+          <p className="text-xs text-zinc-500 px-6 py-2">Loading transactions...</p>
+        ) : txns.length === 0 ? (
+          <p className="text-xs text-zinc-600 px-6 py-2">No transactions found</p>
+        ) : (
+          <table className="w-full text-xs">
+            <thead>
+              <tr className="text-zinc-600">
+                <th className="text-left px-6 py-1 font-normal w-24">Date</th>
+                <th className="text-left px-2 py-1 font-normal">Description</th>
+                <th className="text-right px-2 py-1 font-normal w-24">Amount</th>
+                <th className="text-right px-4 py-1 font-normal w-36">Category</th>
+              </tr>
+            </thead>
+            <tbody>
+              {txns.map((tx) => (
+                <tr key={tx.id} className="border-t border-zinc-800/30 hover:bg-zinc-800/20">
+                  <td className="px-6 py-1.5 text-zinc-500 tabular-nums">
+                    {tx.transaction_date.slice(5).replace("-", "/")}
+                  </td>
+                  <td className="px-2 py-1.5 text-zinc-300 max-w-xs truncate">
+                    {tx.effective_merchant || tx.description}
+                  </td>
+                  <td className="px-2 py-1.5 text-right tabular-nums text-zinc-300">
+                    {fmtExact(tx.amount)}
+                  </td>
+                  <td className="px-4 py-1.5 text-right">
+                    <select
+                      className="bg-zinc-800 border border-zinc-700 rounded px-2 py-0.5 text-xs text-zinc-300 focus:outline-none focus:border-indigo-500 cursor-pointer"
+                      defaultValue={tx.effective_category}
+                      onChange={(e) => handleCategoryChange(tx.id, e.target.value)}
+                    >
+                      {CATEGORIES.map((cat) => (
+                        <option key={cat} value={cat}>
+                          {formatCategory(cat)}
+                        </option>
+                      ))}
+                    </select>
+                  </td>
+                </tr>
+              ))}
+            </tbody>
+          </table>
+        )}
+      </td>
+    </tr>
+  );
+}
+
 export default function AnalyticsPage() {
   const [selectedMonth, setSelectedMonth] = useState(currentMonthStr);
+  const [expandedCategory, setExpandedCategory] = useState<string | null>(null);
   const { data: analytics, isLoading } = useMonthlyAnalytics(6);
 
   if (isLoading || !analytics) {
@@ -96,22 +255,34 @@ export default function AnalyticsPage() {
 
   const largestCategory = categoryRows[0];
 
-  // Cashflow bar chart data (chronological)
-  const cashflowData = months.map((m) => ({
-    month: m,
-    label: formatShortMonth(m),
-    income: analytics.totals[m]?.income || 0,
-    expenses: analytics.totals[m]?.spent || 0,
-    investments: analytics.totals[m]?.investments || 0,
-  }));
-
-  // Category horizontal bar chart
-  const categoryChartData = [...categoryRows].reverse();
-  const chartHeight = Math.max(categoryChartData.length * 36, 120);
-
   const hasIncome = months.some((m) => (analytics.totals[m]?.income || 0) > 0);
   const hasInvestments = months.some((m) => (analytics.totals[m]?.investments || 0) > 0);
 
+  // Categories that appear in expense data
+  const expenseCategories = analytics.rows.map((r) => r.category);
+
+  // Cashflow chart data (chronological) — per-category amounts for stacking
+  const cashflowData = months.map((m) => {
+    const entry: Record<string, unknown> = {
+      month: m,
+      label: formatShortMonth(m),
+      income: analytics.income[m] || 0,
+    };
+    analytics.rows.forEach((row) => {
+      entry[`cat_${row.category}`] = row.spent[m] || 0;
+    });
+    const income = analytics.income[m] || 0;
+    const spent = analytics.totals[m]?.spent || 0;
+    const invested = analytics.investments[m] || 0;
+    entry.savingsRate =
+      income > 0 ? Math.round(((income - spent - invested) / income) * 100) : null;
+    return entry;
+  });
+
+  // Horizontal category bar chart for selected month
+  const categoryChartData = [...categoryRows].reverse();
+  const chartHeight = Math.max(categoryChartData.length * 36, 120);
+
   return (
     <div className="space-y-6">
       {/* Header + month selector */}
@@ -136,7 +307,7 @@ export default function AnalyticsPage() {
         </div>
       </div>
 
-      {/* Cashflow summary cards */}
+      {/* Summary cards */}
       <div className={`grid gap-4 ${hasIncome ? "grid-cols-2 sm:grid-cols-4" : "grid-cols-3"}`}>
         {hasIncome && (
           <div className="bg-zinc-900 border border-zinc-700 rounded-xl p-4">
@@ -149,9 +320,11 @@ export default function AnalyticsPage() {
           <p className="text-xs text-zinc-500 mb-1">Expenses</p>
           <p className="text-2xl font-semibold">{fmtExact(totals.spent)}</p>
           <p className={`text-xs mt-1 ${deltaColor(spentDelta)}`}>
-            {spentDelta === 0 || lastTotals.spent === 0
+            {spentDelta === 0 || lastTotals.spent === 0 ? (
-              ? <span className="text-zinc-500">split-adjusted</span>
+              <span className="text-zinc-500">split-adjusted</span>
-              : `${spentDelta > 0 ? "+" : ""}${fmtExact(spentDelta)} vs ${formatShortMonth(prevMonth(selectedMonth))}`}
+            ) : (
+              `${spentDelta > 0 ? "+" : ""}${fmtExact(spentDelta)} vs ${formatShortMonth(prevMonth(selectedMonth))}`
+            )}
           </p>
         </div>
         {(hasInvestments || totals.investments > 0) && (
@@ -165,8 +338,11 @@ export default function AnalyticsPage() {
           <p className="text-xs text-zinc-500 mb-1">{hasIncome ? "Net Cash" : "Largest Category"}</p>
           {hasIncome ? (
             <>
-              <p className={`text-2xl font-semibold ${totals.net >= 0 ? "text-emerald-400" : "text-red-400"}`}>
+              <p
-                {totals.net >= 0 ? "+" : ""}{fmtExact(totals.net)}
+                className={`text-2xl font-semibold ${totals.net >= 0 ? "text-emerald-400" : "text-red-400"}`}
+              >
+                {totals.net >= 0 ? "+" : ""}
+                {fmtExact(totals.net)}
               </p>
               <p className="text-xs text-zinc-500 mt-1">income − expenses − invested</p>
             </>
@@ -181,13 +357,36 @@ export default function AnalyticsPage() {
         </div>
       </div>
 
-      {/* Cashflow bar chart — income vs expenses vs investments */}
+      {/* Cashflow chart — stacked expense categories + optional savings rate line */}
       <div className="bg-zinc-900 border border-zinc-700 rounded-xl p-4">
-        <h3 className="text-sm font-medium mb-4">
+        <div className="flex items-center justify-between mb-4">
-          {hasIncome ? "Cashflow — 6 Months" : "Monthly Spending"}
+          <h3 className="text-sm font-medium">
+            {hasIncome ? "Cashflow — 6 Months" : "Monthly Spending by Category"}
           </h3>
-        <ResponsiveContainer width="100%" height={220}>
+          {hasIncome && (
-          <BarChart data={cashflowData} margin={{ top: 0, right: 8, bottom: 0, left: 8 }}>
+            <div className="flex items-center gap-3 text-xs text-zinc-500">
+              <span className="flex items-center gap-1">
+                <span className="w-3 h-0.5 bg-amber-400 inline-block" />
+                Savings Rate
+              </span>
+              <span className="flex items-center gap-1">
+                <span className="w-2.5 h-2.5 rounded-sm bg-emerald-500 inline-block" />
+                Income
+              </span>
+            </div>
+          )}
+        </div>
+        <ResponsiveContainer width="100%" height={240}>
+          <ComposedChart
+            data={cashflowData}
+            margin={{ top: 4, right: hasIncome ? 48 : 8, bottom: 0, left: 8 }}
+            onClick={(data) => {
+              // eslint-disable-next-line @typescript-eslint/no-explicit-any
+              const month = (data as any)?.activePayload?.[0]?.payload?.month as string | undefined;
+              if (month) setSelectedMonth(month);
+            }}
+            style={{ cursor: "pointer" }}
+          >
             <XAxis
               dataKey="label"
               tick={{ fill: "#71717a", fontSize: 12 }}
@@ -195,43 +394,92 @@ export default function AnalyticsPage() {
               tickLine={false}
             />
             <YAxis
+              yAxisId="left"
               tick={{ fill: "#71717a", fontSize: 11 }}
               axisLine={false}
               tickLine={false}
               tickFormatter={(v) => `$${v}`}
               width={56}
             />
-            <Tooltip
-              formatter={(value, name) => [fmtExact(Number(value)), name === "expenses" ? "Expenses" : name === "income" ? "Income" : "Invested"]}
-              contentStyle={TOOLTIP_STYLE}
-              labelStyle={{ color: "#a1a1aa" }}
-              cursor={{ fill: "rgba(255,255,255,0.04)" }}
-            />
             {hasIncome && (
-              <>
+              <YAxis
-                <Legend
+                yAxisId="right"
-                  formatter={(value) => value === "income" ? "Income" : value === "expenses" ? "Expenses" : "Invested"}
+                orientation="right"
-                  wrapperStyle={{ fontSize: 12, color: "#71717a" }}
+                tick={{ fill: "#71717a", fontSize: 11 }}
+                axisLine={false}
+                tickLine={false}
+                tickFormatter={(v) => `${v}%`}
+                domain={[-20, 100]}
+                width={36}
               />
-                <Bar dataKey="income" fill="#34d399" radius={[4, 4, 0, 0]} />
-                {hasInvestments && <Bar dataKey="investments" fill="#818cf8" radius={[4, 4, 0, 0]} />}
-              </>
             )}
+            <Tooltip content={<CashflowTooltip />} cursor={{ fill: "rgba(255,255,255,0.04)" }} />
+
+            {/* Income bar (ungrouped, green) */}
+            {hasIncome && (
               <Bar
-              dataKey="expenses"
+                yAxisId="left"
-              radius={[4, 4, 0, 0]}
+                dataKey="income"
-              onClick={(data) => setSelectedMonth((data as unknown as { month: string }).month)}
+                fill="#22c55e"
-              style={{ cursor: "pointer" }}
+                radius={[3, 3, 0, 0]}
+                maxBarSize={32}
+                opacity={0.85}
+              />
+            )}
+
+            {/* Stacked expense category bars */}
+            {expenseCategories.map((cat, i) => (
+              <Bar
+                key={cat}
+                yAxisId="left"
+                dataKey={`cat_${cat}`}
+                stackId="expenses"
+                fill={CATEGORY_COLORS[cat] || "#71717a"}
+                radius={i === expenseCategories.length - 1 ? [3, 3, 0, 0] : [0, 0, 0, 0]}
+                maxBarSize={32}
               >
                 {cashflowData.map((entry) => (
                   <Cell
-                  key={entry.month}
+                    key={entry.month as string}
-                  fill={entry.month === selectedMonth ? "#6366f1" : (hasIncome ? "#ef4444" : "#3f3f46")}
+                    fill={
+                      entry.month === selectedMonth
+                        ? CATEGORY_COLORS[cat] || "#71717a"
+                        : `${CATEGORY_COLORS[cat] || "#71717a"}99`
+                    }
                   />
                 ))}
               </Bar>
-          </BarChart>
+            ))}
+
+            {/* Savings rate line */}
+            {hasIncome && (
+              <Line
+                yAxisId="right"
+                dataKey="savingsRate"
+                stroke="#fbbf24"
+                strokeWidth={2}
+                dot={{ fill: "#fbbf24", r: 3 }}
+                activeDot={{ r: 5 }}
+                connectNulls={false}
+              />
+            )}
+          </ComposedChart>
         </ResponsiveContainer>
+
+        {/* Category color legend */}
+        {expenseCategories.length > 0 && (
+          <div className="flex flex-wrap gap-x-3 gap-y-1 mt-3 pt-3 border-t border-zinc-800">
+            {expenseCategories.map((cat) => (
+              <span key={cat} className="flex items-center gap-1 text-xs text-zinc-500">
+                <span
+                  className="w-2 h-2 rounded-sm shrink-0"
+                  style={{ background: CATEGORY_COLORS[cat] || "#71717a" }}
+                />
+                {formatCategory(cat)}
+              </span>
+            ))}
+          </div>
+        )}
       </div>
 
       {/* Spending by category — horizontal bar chart */}
@@ -269,17 +517,26 @@ export default function AnalyticsPage() {
                 labelStyle={{ color: "#a1a1aa" }}
                 cursor={{ fill: "rgba(255,255,255,0.04)" }}
               />
-              <Bar dataKey="spent" fill="#6366f1" radius={[0, 4, 4, 0]} />
+              <Bar dataKey="spent" radius={[0, 4, 4, 0]}>
+                {categoryChartData.map((entry) => (
+                  <Cell
+                    key={entry.category}
+                    fill={CATEGORY_COLORS[entry.category] || "#6366f1"}
+                  />
+                ))}
+              </Bar>
             </BarChart>
           </ResponsiveContainer>
         </div>
       )}
 
-      {/* Category breakdown table */}
+      {/* Category breakdown table — with expandable rows */}
       {categoryRows.length > 0 && (
         <div className="bg-zinc-900 border border-zinc-700 rounded-xl overflow-hidden">
           <div className="px-4 py-3 border-b border-zinc-800">
-            <h3 className="text-sm font-medium">Spending Breakdown — {formatMonth(selectedMonth)}</h3>
+            <h3 className="text-sm font-medium">
+              Spending Breakdown — {formatMonth(selectedMonth)}
+            </h3>
           </div>
           <table className="w-full text-sm">
             <thead>
@@ -291,16 +548,47 @@ export default function AnalyticsPage() {
               </tr>
             </thead>
             <tbody>
-              {categoryRows.map(({ category, spent, txCount }) => (
+              {categoryRows.map(({ category, spent, txCount }) => {
-                <tr key={category} className="border-b border-zinc-800/50 hover:bg-zinc-800/30">
+                const isExpanded = expandedCategory === category;
-                  <td className="px-4 py-2.5 font-medium">{formatCategory(category)}</td>
+                return (
+                  <>
+                    <tr
+                      key={category}
+                      className={`border-b border-zinc-800/50 cursor-pointer select-none transition-colors ${
+                        isExpanded ? "bg-zinc-800/40" : "hover:bg-zinc-800/30"
+                      }`}
+                      onClick={() =>
+                        setExpandedCategory(isExpanded ? null : category)
+                      }
+                    >
+                      <td className="px-4 py-2.5 font-medium">
+                        <span className="flex items-center gap-2">
+                          <span
+                            className="w-2 h-2 rounded-sm shrink-0"
+                            style={{ background: CATEGORY_COLORS[category] || "#71717a" }}
+                          />
+                          {formatCategory(category)}
+                          <span className="text-zinc-600 text-xs ml-1">
+                            {isExpanded ? "▲" : "▼"}
+                          </span>
+                        </span>
+                      </td>
                       <td className="px-4 py-2.5 text-right tabular-nums">{fmtExact(spent)}</td>
                       <td className="px-4 py-2.5 text-right text-zinc-400">{txCount}</td>
                       <td className="px-4 py-2.5 text-right text-zinc-400 tabular-nums">
                         {totals.spent > 0 ? ((spent / totals.spent) * 100).toFixed(1) : "0.0"}%
                       </td>
                     </tr>
-              ))}
+                    {isExpanded && (
+                      <CategoryPanel
+                        key={`panel-${category}`}
+                        category={category}
+                        selectedMonth={selectedMonth}
+                      />
+                    )}
+                  </>
+                );
+              })}
             </tbody>
           </table>
         </div>
@@ -334,7 +622,13 @@ export default function AnalyticsPage() {
                 {analytics.rows.map((row) => (
                   <tr key={row.category} className="border-b border-zinc-800/40 hover:bg-zinc-900/30">
                     <td className="px-3 py-2 font-medium sticky left-0 bg-zinc-950">
+                      <span className="flex items-center gap-1.5">
+                        <span
+                          className="w-1.5 h-1.5 rounded-sm shrink-0"
+                          style={{ background: CATEGORY_COLORS[row.category] || "#71717a" }}
+                        />
                         {formatCategory(row.category)}
+                      </span>
                     </td>
                     {analytics.months.map((m) => {
                       const spent = row.spent[m];
@@ -361,7 +655,9 @@ export default function AnalyticsPage() {
                       return (
                         <td
                           key={m}
-                          className={`px-3 py-2 text-right tabular-nums text-emerald-500 ${m === selectedMonth ? "bg-zinc-800/30" : ""}`}
+                          className={`px-3 py-2 text-right tabular-nums text-emerald-500 ${
+                            m === selectedMonth ? "bg-zinc-800/30" : ""
+                          }`}
                         >
                           {inc ? fmt(inc) : "—"}
                         </td>
@@ -379,7 +675,9 @@ export default function AnalyticsPage() {
                       return (
                         <td
                           key={m}
-                          className={`px-3 py-2 text-right tabular-nums text-indigo-400 ${m === selectedMonth ? "bg-zinc-800/30" : ""}`}
+                          className={`px-3 py-2 text-right tabular-nums text-indigo-400 ${
+                            m === selectedMonth ? "bg-zinc-800/30" : ""
+                          }`}
                         >
                           {inv ? fmt(inv) : "—"}
                         </td>
@@ -416,7 +714,8 @@ export default function AnalyticsPage() {
                             net >= 0 ? "text-emerald-400" : "text-red-400"
                           } ${m === selectedMonth ? "bg-zinc-800/30" : ""}`}
                         >
-                          {net >= 0 ? "+" : ""}{fmt(net)}
+                          {net >= 0 ? "+" : ""}
+                          {fmt(net)}
                         </td>
                       );
                     })}