Add allocation evolution chart showing allocation changes over time

Adds a stacked area chart under the Allocation tab that displays how the
portfolio allocation percentages have changed across months.

Fixes #25

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

PortfolioJournal/ViewModels/ChartsViewModel.swift

@@ -103,6 +103,7 @@ class ChartsViewModel: ObservableObject {
     @Published var drawdownData: [(date: Date, drawdown: Double)] = []
     @Published var volatilityData: [(date: Date, volatility: Double)] = []
     @Published var predictionData: [Prediction] = []
+    @Published var allocationEvolutionData: [(date: Date, category: String, percentage: Double, color: String)] = []
 
     @Published var isLoading = false
     @Published var showingPaywall = false
@@ -340,6 +341,7 @@ class ChartsViewModel: ObservableObject {
             calculateCategoryEvolutionData(from: completedSnapshots, categories: categoriesForChart)
         case .allocation:
             calculateAllocationData(for: sources, breakdown: selectedBreakdown)
+            calculateAllocationEvolutionData(from: completedSnapshots)
         case .performance:
             let completedSnapshotsBySource = groupSnapshotsBySource(completedSnapshots)
             calculatePerformanceData(
@@ -587,6 +589,57 @@ class ChartsViewModel: ObservableObject {
         }
     }
 
+    private func calculateAllocationEvolutionData(from snapshots: [Snapshot]) {
+        let sortedSnapshots = snapshots.sorted { $0.date < $1.date }
+        let groupedByMonth = Dictionary(grouping: sortedSnapshots) { snapshot -> DateComponents in
+            Calendar.current.dateComponents([.year, .month], from: snapshot.date)
+        }
+
+        let sortedMonths = groupedByMonth.keys.sorted {
+            let d1 = ($0.year ?? 0) * 100 + ($0.month ?? 0)
+            let d2 = ($1.year ?? 0) * 100 + ($1.month ?? 0)
+            return d1 < d2
+        }
+
+        var result: [(date: Date, category: String, percentage: Double, color: String)] = []
+
+        for monthKey in sortedMonths {
+            guard let monthSnapshots = groupedByMonth[monthKey],
+                  let monthDate = Calendar.current.date(from: monthKey) else { continue }
+
+            var categoryTotals: [String: (value: Decimal, color: String)] = [:]
+            var sourceLatest: [UUID: Snapshot] = [:]
+
+            for snapshot in monthSnapshots {
+                guard let sourceId = snapshot.source?.id else { continue }
+                if let existing = sourceLatest[sourceId] {
+                    if snapshot.date > existing.date {
+                        sourceLatest[sourceId] = snapshot
+                    }
+                } else {
+                    sourceLatest[sourceId] = snapshot
+                }
+            }
+
+            for (_, snapshot) in sourceLatest {
+                let categoryName = snapshot.source?.category?.name ?? "Other"
+                let colorHex = snapshot.source?.category?.colorHex ?? "#6B7280"
+                let existing = categoryTotals[categoryName] ?? (value: 0, color: colorHex)
+                categoryTotals[categoryName] = (value: existing.value + snapshot.decimalValue, color: colorHex)
+            }
+
+            let total = categoryTotals.values.reduce(Decimal.zero) { $0 + $1.value }
+            guard total > 0 else { continue }
+
+            for (category, info) in categoryTotals.sorted(by: { $0.value.value > $1.value.value }) {
+                let percentage = NSDecimalNumber(decimal: info.value / total * 100).doubleValue
+                result.append((date: monthDate, category: category, percentage: percentage, color: info.color))
+            }
+        }
+
+        allocationEvolutionData = result
+    }
+
     private func completedMonthKeys(
         sources: [InvestmentSource],
         snapshots: [Snapshot],

PortfolioJournal/Views/Charts/ChartsContainerView.swift

@@ -308,11 +308,17 @@ struct ChartsContainerView: View {
                     goals: goalsViewModel.goals
                 )
             case .allocation:
+                VStack(spacing: 20) {
                     AllocationPieChart(
                         data: viewModel.allocationData,
                         title: viewModel.selectedBreakdown == .source ? "Allocation by Source" : "Asset Allocation",
                         showsTargetsComparison: ChartsViewModel.supportsAllocationTargets(for: viewModel.selectedBreakdown)
                     )
+
+                    if !viewModel.allocationEvolutionData.isEmpty {
+                        AllocationEvolutionChart(data: viewModel.allocationEvolutionData)
+                    }
+                }
             case .performance:
                 PerformanceBarChart(
                     data: viewModel.performanceData,
@@ -920,6 +926,70 @@ struct CashflowStackedChartView: View {
     }
 }
 
+// MARK: - Allocation Evolution Chart
+
+struct AllocationEvolutionChart: View {
+    let data: [(date: Date, category: String, percentage: Double, color: String)]
+
+    var body: some View {
+        VStack(alignment: .leading, spacing: 12) {
+            Text("Allocation Over Time")
+                .font(.headline)
+
+            if data.isEmpty {
+                Text("Not enough data to show allocation evolution.")
+                    .foregroundColor(.secondary)
+                    .frame(height: 260)
+            } else {
+                Chart {
+                    ForEach(data, id: \.date.description + \.category) { item in
+                        AreaMark(
+                            x: .value("Date", item.date),
+                            y: .value("Percentage", item.percentage)
+                        )
+                        .foregroundStyle(by: .value("Category", item.category))
+                    }
+                }
+                .chartForegroundStyleScale(domain: categoryNames, range: categoryColors)
+                .chartXAxis {
+                    AxisMarks(values: .stride(by: .month, count: 2)) { _ in
+                        AxisValueLabel(format: .dateTime.month(.abbreviated).year(.twoDigits))
+                    }
+                }
+                .chartYAxis {
+                    AxisMarks(position: .leading) { value in
+                        AxisValueLabel {
+                            if let pct = value.as(Double.self) {
+                                Text(String(format: "%.0f%%", pct))
+                                    .font(.caption)
+                            }
+                        }
+                    }
+                }
+                .frame(height: 260)
+                .drawingGroup()
+            }
+        }
+        .padding()
+        .background(Color(.systemBackground))
+        .cornerRadius(AppConstants.UI.cornerRadius)
+        .shadow(color: .black.opacity(0.05), radius: 8, y: 2)
+    }
+
+    private var categoryNames: [String] {
+        Array(Set(data.map { $0.category })).sorted()
+    }
+
+    private var categoryColors: [Color] {
+        categoryNames.map { name in
+            if let hex = data.first(where: { $0.category == name })?.color {
+                return Color(hex: hex) ?? .gray
+            }
+            return .gray
+        }
+    }
+}
+
 #Preview {
     ChartsContainerView(iapService: IAPService())
         .environmentObject(AccountStore(iapService: IAPService()))