Backend/ERPCore/Dtos/Dashboard/DashboardDtos.cs

@@ -23,7 +23,12 @@ public sealed record DashboardStatsDto(
     int PendingCounts,
     int OpenRfqs,
     int PendingLeaveRequests,
-    int OverdueSalesInvoices);
+    int OverdueSalesInvoices,
+    IReadOnlyList<MonthlyPurchasesVsSalesDto> MonthlyPurchasesVsSales);
 
 /// <summary>One bar in the Stock Valuation chart — total FIFO layer value for a warehouse.</summary>
 public sealed record WarehouseValuationDto(int WarehouseId, decimal Total);
+
+/// <summary>One month in the Monthly Purchases chart (Purchase Invoices page) — GrandTotal
+/// of Posted purchase invoices vs Posted sales invoices. <c>Month</c> is "yyyy-MM".</summary>
+public sealed record MonthlyPurchasesVsSalesDto(string Month, decimal Purchases, decimal Sales);

Backend/ERPCore/Services/DashboardService.cs

@@ -26,6 +26,7 @@ public sealed class DashboardService : IDashboardService
     private readonly IRepository<Rfq> _rfqs;
     private readonly IRepository<LeaveRequest> _leaveRequests;
     private readonly IRepository<SalesInvoice> _salesInvoices;
+    private readonly IRepository<PurchaseInvoice> _purchaseInvoices;
     private readonly IReorderService _reorder;
 
     // Neither SalesInvoice nor PurchaseInvoice carries a DueDate field (Phase 1 schema
@@ -33,11 +34,14 @@ public sealed class DashboardService : IDashboardService
     // balance older than this many days from its invoice date.
     private const int OverdueAgeDays = 30;
 
+    // Monthly Purchases chart window: the current month plus the previous 11.
+    private const int ChartMonths = 12;
+
     public DashboardService(
         IRepository<StockLayer> layers, IRepository<Grn> grns, IRepository<PurchaseOrder> pos,
         IRepository<Requisition> requisitions, IRepository<StockCount> counts, IRepository<Rfq> rfqs,
         IRepository<LeaveRequest> leaveRequests, IRepository<SalesInvoice> salesInvoices,
-        IReorderService reorder)
+        IRepository<PurchaseInvoice> purchaseInvoices, IReorderService reorder)
     {
         _layers = layers;
         _grns = grns;
@@ -47,6 +51,7 @@ public sealed class DashboardService : IDashboardService
         _rfqs = rfqs;
         _leaveRequests = leaveRequests;
         _salesInvoices = salesInvoices;
+        _purchaseInvoices = purchaseInvoices;
         _reorder = reorder;
     }
 
@@ -103,9 +108,50 @@ public sealed class DashboardService : IDashboardService
         var lowStockAlerts = (await _reorder.GetAlertsAsync(null, branchId, new PageQuery { Page = 1, PageSize = 1 }, ct))
             .Pagination.TotalItems;
 
+        var monthlyPurchasesVsSales = await GetMonthlyPurchasesVsSalesAsync(branchId, salesInvoices, ct);
+
         return new DashboardStatsDto(
             lowStockAlerts, onHandTotal, onHandWarehouses, stockValuationTotal, stockValuationByWarehouse,
             pendingApprovalPOs, pendingGrns, openRequisitions, pendingCounts, openRfqs,
-            pendingLeaveRequests, overdueSalesInvoices);
+            pendingLeaveRequests, overdueSalesInvoices, monthlyPurchasesVsSales);
+    }
+
+    /// <summary>
+    /// Posted purchase vs sales invoice GrandTotals per calendar month (UTC) over the last
+    /// <see cref="ChartMonths"/> months, oldest first. Months with neither are omitted, so an
+    /// empty list means "no posted invoices yet". Purchases are dated by VendorInvoiceDate,
+    /// sales by InvoiceDate.
+    /// </summary>
+    private async Task<IReadOnlyList<MonthlyPurchasesVsSalesDto>> GetMonthlyPurchasesVsSalesAsync(
+        int? branchId, IQueryable<SalesInvoice> salesInvoices, CancellationToken ct)
+    {
+        var now = DateTime.UtcNow;
+        var since = new DateTime(now.Year, now.Month, 1, 0, 0, 0, DateTimeKind.Utc).AddMonths(-(ChartMonths - 1));
+
+        var purchaseInvoices = _purchaseInvoices.Query().AsNoTracking();
+        if (branchId is not null) purchaseInvoices = purchaseInvoices.Where(i => i.BranchId == branchId);
+        var purchases = await purchaseInvoices
+            .Where(i => i.Status == PurchaseInvoiceStatus.Posted && i.VendorInvoiceDate >= since)
+            .GroupBy(i => new { i.VendorInvoiceDate.Year, i.VendorInvoiceDate.Month })
+            .Select(g => new { g.Key.Year, g.Key.Month, Total = g.Sum(i => i.GrandTotal) })
+            .ToListAsync(ct);
+
+        var sales = await salesInvoices
+            .Where(i => i.Status == SalesInvoiceStatus.Posted && i.InvoiceDate >= since)
+            .GroupBy(i => new { i.InvoiceDate.Year, i.InvoiceDate.Month })
+            .Select(g => new { g.Key.Year, g.Key.Month, Total = g.Sum(i => i.GrandTotal) })
+            .ToListAsync(ct);
+
+        var purchasesByMonth = purchases.ToDictionary(p => (p.Year, p.Month), p => p.Total);
+        var salesByMonth = sales.ToDictionary(s => (s.Year, s.Month), s => s.Total);
+
+        return Enumerable.Range(0, ChartMonths)
+            .Select(offset => since.AddMonths(offset))
+            .Select(m => new MonthlyPurchasesVsSalesDto(
+                $"{m.Year:D4}-{m.Month:D2}",
+                purchasesByMonth.GetValueOrDefault((m.Year, m.Month)),
+                salesByMonth.GetValueOrDefault((m.Year, m.Month))))
+            .Where(m => m.Purchases != 0m || m.Sales != 0m)
+            .ToList();
     }
 }

Frontend/erp-system/types/dashboard.ts

@@ -19,4 +19,13 @@ export interface DashboardStats {
   openRfqs: number
   pendingLeaveRequests: number
   overdueSalesInvoices: number
+  monthlyPurchasesVsSales: MonthlyPurchasesVsSales[]
+}
+
+// One month of Posted purchase vs sales invoice GrandTotals; `month` is "yyyy-MM".
+// Only months with activity are returned (last 12, oldest first).
+export interface MonthlyPurchasesVsSales {
+  month: string
+  purchases: number
+  sales: number
 }