diff --git a/frontend/apps/finance/src/features/dashboard/__tests__/CumulativeSpendChart.test.tsx b/frontend/apps/finance/src/features/dashboard/__tests__/CumulativeSpendChart.test.tsx
new file mode 100644
index 0000000..af786c1
--- /dev/null
+++ b/frontend/apps/finance/src/features/dashboard/__tests__/CumulativeSpendChart.test.tsx
@@ -0,0 +1,69 @@
+import { describe, it, expect, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import {
+  CumulativeSpendChart,
+} from "../components/widgets/CumulativeSpendChart";
+import {
+  tooltipFormatter,
+  tooltipLabelFormatter,
+} from "../components/widgets/cumulative-spend-chart-utils";
+
+// ResponsiveContainer won't render children without real DOM dimensions.
+// Mock it to render children directly so chart internals execute.
+vi.mock("recharts", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("recharts")>();
+  return {
+    ...actual,
+    ResponsiveContainer: ({ children }: { children: React.ReactNode }) => (
+      <div style={{ width: 800, height: 300 }}>{children}</div>
+    ),
+  };
+});
+
+// Data that produces a crossover: actual starts above ideal, then drops below
+const testData = [
+  { day: 1, actual: 20000, ideal: 10000 },
+  { day: 2, actual: 35000, ideal: 20000 },
+  { day: 3, actual: 40000, ideal: 30000 },
+  { day: 4, actual: 42000, ideal: 40000 },
+  { day: 5, actual: 44000, ideal: 50000 },
+  { day: 6, actual: 46000, ideal: 60000 },
+  { day: 7, actual: 48000, ideal: 70000 },
+];
+
+describe("CumulativeSpendChart", () => {
+  it("renders chart card with title", () => {
+    render(<CumulativeSpendChart data={testData} currency="GBP" />);
+    expect(screen.getByText("Cumulative Spending")).toBeInTheDocument();
+  });
+
+  it("renders with empty data without crashing", () => {
+    render(<CumulativeSpendChart data={[]} currency="USD" />);
+    expect(screen.getByText("Cumulative Spending")).toBeInTheDocument();
+  });
+});
+
+describe("tooltipFormatter", () => {
+  it("returns null for array values (range area tuples)", () => {
+    expect(tooltipFormatter([100, 200], "surplus", "GBP")).toBeNull();
+  });
+
+  it("formats scalar values as currency", () => {
+    const result = tooltipFormatter(150, "Actual", "GBP");
+    expect(result).not.toBeNull();
+    expect(result![0]).toContain("£");
+    expect(result![1]).toBe("Actual");
+  });
+});
+
+describe("tooltipLabelFormatter", () => {
+  it("formats integer day labels", () => {
+    expect(tooltipLabelFormatter(5)).toBe("Day 5");
+    expect(tooltipLabelFormatter(17)).toBe("Day 17");
+  });
+
+  it("returns empty string for fractional crossover values", () => {
+    expect(tooltipLabelFormatter(17.16)).toBe("");
+    expect(tooltipLabelFormatter(4.5)).toBe("");
+  });
+});
diff --git a/frontend/apps/finance/src/features/dashboard/components/widgets/CumulativeSpendChart.tsx b/frontend/apps/finance/src/features/dashboard/components/widgets/CumulativeSpendChart.tsx
index e10b8ba..cd16ba6 100644
--- a/frontend/apps/finance/src/features/dashboard/components/widgets/CumulativeSpendChart.tsx
+++ b/frontend/apps/finance/src/features/dashboard/components/widgets/CumulativeSpendChart.tsx
@@ -1,6 +1,10 @@
-import { formatCurrency, getCurrencySymbol } from "@gofin/core";
+import { getCurrencySymbol } from "@gofin/core";
 import type { CumulativeSpendPoint } from "../../../../types";
 import { insertCrossoverPoints } from "../../../../lib/insertCrossoverPoints";
+import {
+  tooltipFormatter,
+  tooltipLabelFormatter,
+} from "./cumulative-spend-chart-utils";
 import {
   Card,
   CardContent,
@@ -40,13 +44,13 @@ export function CumulativeSpendChart({ data, currency }: CumulativeSpendChartPro
     const underBudget = point.actual <= point.ideal;
 
     if (point.isCrossover) {
-      // At crossover, define both areas to terminate/start seamlessly
+      // At crossover, both areas collapse to zero height for seamless transition
       return {
         day: point.day,
         actual: point.actual,
         ideal: point.ideal,
-        surplusTop: point.actual,
-        deficitTop: point.actual,
+        surplus: [point.actual, point.actual] as [number, number],
+        deficit: [point.actual, point.actual] as [number, number],
       };
     }
 
@@ -54,8 +58,12 @@ export function CumulativeSpendChart({ data, currency }: CumulativeSpendChartPro
       day: point.day,
       actual: point.actual,
       ideal: point.ideal,
-      surplusTop: underBudget ? point.ideal : undefined,
-      deficitTop: !underBudget ? point.actual : undefined,
+      surplus: underBudget
+        ? ([point.actual, point.ideal] as [number, number])
+        : undefined,
+      deficit: !underBudget
+        ? ([point.ideal, point.actual] as [number, number])
+        : undefined,
     };
   });
 
@@ -77,17 +85,22 @@ export function CumulativeSpendChart({ data, currency }: CumulativeSpendChartPro
             <CartesianGrid strokeDasharray="3 3" />
             <XAxis
               dataKey="day"
+              type="number"
+              domain={[1, basePoints.length]}
+              ticks={basePoints.map((p) => p.day)}
+              allowDecimals={false}
               label={{ value: "Day of Month", position: "insideBottom", offset: -5 }}
             />
             <YAxis
               tickFormatter={(value) => `${getCurrencySymbol(currency)}${value}`}
             />
             <Tooltip
-              formatter={(value) => formatCurrency((value as number) * 100, currency)}
+              formatter={(value, name) => tooltipFormatter(value, name as string, currency)}
+              labelFormatter={(label) => tooltipLabelFormatter(label)}
             />
             <Area
               type="monotone"
-              dataKey="surplusTop"
+              dataKey="surplus"
               fill="rgba(34, 197, 94, 0.50)"
               stroke="none"
               name="Under Budget"
@@ -96,7 +109,7 @@ export function CumulativeSpendChart({ data, currency }: CumulativeSpendChartPro
             />
             <Area
               type="monotone"
-              dataKey="deficitTop"
+              dataKey="deficit"
               fill="rgba(239, 68, 68, 0.50)"
               stroke="none"
               name="Over Budget"
diff --git a/frontend/apps/finance/src/features/dashboard/components/widgets/cumulative-spend-chart-utils.ts b/frontend/apps/finance/src/features/dashboard/components/widgets/cumulative-spend-chart-utils.ts
new file mode 100644
index 0000000..1080eb0
--- /dev/null
+++ b/frontend/apps/finance/src/features/dashboard/components/widgets/cumulative-spend-chart-utils.ts
@@ -0,0 +1,22 @@
+import { formatCurrency } from "@gofin/core";
+
+/**
+ * Tooltip value formatter for the cumulative spend chart.
+ * Returns null for array values (range area tuples) to suppress them in tooltip.
+ */
+export function tooltipFormatter(
+  value: unknown,
+  name: string,
+  currency: string,
+): [string, string] | null {
+  if (Array.isArray(value)) return null;
+  return [formatCurrency((value as number) * 100, currency), name];
+}
+
+/**
+ * Tooltip label formatter that shows "Day N" for integer days
+ * and suppresses display for fractional crossover points.
+ */
+export function tooltipLabelFormatter(label: unknown): string {
+  return Number.isInteger(label) ? `Day ${label}` : "";
+}