fix: hide baseline snapshot from Net Worth Evolution chart

For YTD/1Y/3Y/5Y periods, getSnapshotsForPeriod includes one extra month
before the range as a baseline to enable first-month return calculations.
This baseline was visible as a spurious data point in the Evoluzione
Patrimonio AreaChart (e.g. Dec 2024 appearing in a YTD 2025 chart).

Added optional skipBaseline param to preparePerformanceChartData; caller
(getChartData in page.tsx) passes true for baseline periods. Consistent
with the existing pattern in prepareMonthlyReturnsHeatmap (i=1 loop).

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

Author: GiuseppeDM98

AGENTS.md

@@ -128,6 +128,13 @@ ALL fields in settings types must be handled in THREE places:
 - **Currency**: Use `currencyConversionService.ts` (Frankfurter API, 24h cache)
 - **Stamp Duty (Imposta di Bollo)**: `calculateStampDuty(assets, rate, checkingAccountSubCategory?)` in `assetService.ts`. Excluded: `quantity=0` + `stampDutyExempt=true`. Conti correnti (matching subcategory): apply only if value strictly > €5,000. Configured in Settings (`stampDutyEnabled`, `stampDutyRate`, `checkingAccountSubCategory`).
 
+### Dividend Net Amount Storage
+- `netAmount` is computed at creation time (`grossAmount - taxAmount`) and **stored in Firestore** — metrics read the saved field, never recalculate at runtime
+- Auto-scraped dividends (Borsa Italiana): `taxAmount = grossAmount × 0.26` (flat 26%, hardcoded)
+- Bond coupons (auto-generated): `taxRate = asset.taxRate ?? 26` — correctly applies 12.5% for BTPs
+- Manual entries: `taxAmount` is whatever the user entered; `netAmount` auto-filled if not provided
+- YOC/Current Yield netto: use `div.netAmountEur ?? div.netAmount` (prefer EUR conversion)
+
 ### DividendStats Filter Coupling
 - `DividendStats` makes an **independent** API fetch to `/api/dividends/stats` — it does NOT read from parent filtered state
 - Any filter added to `DividendTrackingTab` **must be explicitly passed** as a prop to `DividendStats` and forwarded to the API
@@ -151,6 +158,9 @@ ALL fields in settings types must be handled in THREE places:
 - `getSnapshotsForPeriod` includes 1 extra month before the period as **baseline** for YTD/1Y/3Y/5Y
 - **`hasBaseline`** in `calculatePerformanceForPeriod`: period dates computed from `sortedSnapshots[1]` (not baseline). Active only for YTD/1Y/3Y/5Y with >= 3 snapshots
 - All metric functions that annualize **must use `calculateMonthsDifference(periodEnd, periodStart)`** — NOT `snapshots.length - 1`
+- **Chart baseline hiding** — each chart function handles it independently:
+  - Heatmap (`prepareMonthlyReturnsHeatmap`): loop starts at `i = 1`
+  - Evoluzione Patrimonio (`preparePerformanceChartData`): optional `skipBaseline=true` → `.slice(1)` after sort. Pass `true` for YTD/1Y/3Y/5Y in `getChartData()`. Baseline is always `sortedSnapshots[0]` after chronological sort → slice is safe.
 
 ---
 

CLAUDE.md

@@ -5,7 +5,7 @@ Net Worth Tracker is a Next.js app for Italian investors to track net worth, ass
 
 ## Current Status
 - Versione stack: Next.js 16, React 19, TypeScript 5, Tailwind v4, Firebase, Vitest, date-fns-tz, @nivo/sankey, @anthropic-ai/sdk, cheerio
-- Ultima implementazione: Colonna "Mese Prec. %" nelle tab Prezzi/Valori Anno Corrente (`AssetPriceHistoryTable`, `assetPriceHistoryUtils`, `types/assets`) (2026-03-05)
+- Ultima implementazione: Fix baseline nascosto nel grafico "Evoluzione Patrimonio" per periodi YTD/1Y/3Y/5Y (`preparePerformanceChartData` + `getChartData`) (2026-03-06)
 - In corso ora: nessuna attività attiva
 
 ## Architecture Snapshot

Draft Release Temp.md

@@ -157,6 +157,8 @@
 
 ## 🐛 Bug Fixes
 
+- Fixed the Net Worth Evolution chart ("Evoluzione Patrimonio") showing an extra data point from outside the selected time period — for example, December 2024 appearing as the first point in a YTD 2025 chart. The chart now correctly starts at the first month of the selected period for YTD, 1Y, 3Y, and 5Y views.
+
 - Fixed asset value history (Current Year Values / Historical Values tabs) showing "0,00€" instead of the correct amount for assets whose monthly snapshot was captured while their quantity was 0 — value is now correctly recalculated from the stored unit price × quantity. Future snapshots also no longer include zero-quantity assets in the per-asset breakdown to prevent recurrence.
 
 - Fixed dividend page asset filter not showing bond assets — bonds are now listed alongside stocks and ETFs in the filter dropdown so you can filter coupon entries by bond

app/dashboard/performance/page.tsx

@@ -330,7 +330,10 @@ export default function PerformancePage() {
       metrics.endDate
     );
 
-    return preparePerformanceChartData(periodSnapshots, metrics.cashFlows);
+    // YTD/1Y/3Y/5Y periods include an extra baseline snapshot before the range;
+    // skip it so the chart starts at the first actual month of the selected period.
+    const hasBaseline = ['YTD', '1Y', '3Y', '5Y'].includes(metrics.timePeriod);
+    return preparePerformanceChartData(periodSnapshots, metrics.cashFlows, hasBaseline);
   };
 
   const [chartData, setChartData] = useState<any[]>([]);

lib/services/performanceService.ts

@@ -1499,16 +1499,30 @@ async function calculateRollingPeriods(
 
 /**
  * Prepare chart data for net worth evolution
+ *
+ * @param skipBaseline - When true, drops the first (baseline) snapshot.
+ *   getSnapshotsForPeriod includes an extra month before YTD/1Y/3Y/5Y periods
+ *   so the first month's return can be calculated, but that month falls outside
+ *   the selected period and should not appear as a chart data point.
  */
 export function preparePerformanceChartData(
   snapshots: MonthlySnapshot[],
-  cashFlows: CashFlowData[]
+  cashFlows: CashFlowData[],
+  skipBaseline = false
 ): PerformanceChartData[] {
   const sortedSnapshots = [...snapshots].sort((a, b) => {
     if (a.year !== b.year) return a.year - b.year;
     return a.month - b.month;
   });
 
+  // Skip the first snapshot when it is a baseline month (e.g., Dec for YTD).
+  // getSnapshotsForPeriod includes it for return calculations but it falls
+  // outside the selected period and should not appear as a chart data point.
+  const chartSnapshots =
+    skipBaseline && sortedSnapshots.length > 1
+      ? sortedSnapshots.slice(1)
+      : sortedSnapshots;
+
   let cumulativeContributions = 0;
   const cashFlowMap = new Map<string, number>();
 
@@ -1517,7 +1531,7 @@ export function preparePerformanceChartData(
     cashFlowMap.set(key, cf.netCashFlow);
   });
 
-  return sortedSnapshots.map(snapshot => {
+  return chartSnapshots.map(snapshot => {
     const key = `${snapshot.year}-${String(snapshot.month).padStart(2, '0')}`;
     const cashFlow = cashFlowMap.get(key) || 0;
     cumulativeContributions += cashFlow;