diff --git a/packages/buckaroo-js-core/src/components/DFViewerParts/ChartCell.render.test.tsx b/packages/buckaroo-js-core/src/components/DFViewerParts/ChartCell.render.test.tsx
new file mode 100644
index 000000000..d627027cd
--- /dev/null
+++ b/packages/buckaroo-js-core/src/components/DFViewerParts/ChartCell.render.test.tsx
@@ -0,0 +1,77 @@
+/**
+ * Sparkline rendering contract for the chart cell (pinned rows and data
+ * columns):
+ *  - every line series draws at 1px with per-point dots disabled — the
+ *    100x24 chart gets CSS-scaled to fill its grid cell, which turns the
+ *    recharts defaults (dots + scaled stroke) into a fat blobby squiggle
+ *  - a hidden YAxis pads the plot by 2px top and bottom so a series whose
+ *    values sit exactly at dataMin/dataMax (e.g. the "after" side of a
+ *    diff whose values collapsed) isn't drawn on the plot border where
+ *    the clip rect swallows half the stroke
+ */
+jest.mock("react", () => {
+    const actual = jest.requireActual("react");
+    return { __esModule: true, default: actual, ...actual };
+});
+
+import { render } from "@testing-library/react";
+import { getChartCell, LineObservation } from "./ChartCell";
+
+// recharts pulls in DOM-measurement code that doesn't run cleanly under
+// jsdom — stub the exports ChartCell touches, keeping Line/YAxis props
+// inspectable in the rendered tree.
+jest.mock("recharts", () => {
+    const React = require("react");
+    return {
+        Area: () => null,
+        Bar: () => null,
+        Line: ({ dataKey, dot, strokeWidth }: any) =>
+            React.createElement("div", {
+                "data-testid": `line-${dataKey}`,
+                "data-dot": String(dot),
+                "data-stroke-width": String(strokeWidth),
+            }),
+        Tooltip: () => null,
+        YAxis: ({ hide, padding }: any) =>
+            React.createElement("div", {
+                "data-testid": "yaxis-mock",
+                "data-hide": String(hide),
+                "data-padding-top": String(padding?.top),
+                "data-padding-bottom": String(padding?.bottom),
+            }),
+        ComposedChart: ({ children }: any) =>
+            React.createElement("div", { "data-testid": "composedchart-mock" }, children),
+    };
+});
+
+const ChartCell = getChartCell({ displayer: "chart" });
+
+const validChart: LineObservation[] = [{ lineRed: 10 }, { lineRed: 20 }];
+
+const mkProps = (value: any) => ({
+    value,
+    api: {} as any,
+    colDef: { cellClass: "" } as any,
+    column: {} as any,
+    context: {},
+});
+
+describe("ChartCell sparkline rendering", () => {
+    it("draws every line series at 1px with dots disabled", () => {
+        const { container } = render(<ChartCell {...mkProps(validChart)} />);
+        const lines = container.querySelectorAll('[data-testid^="line-"]');
+        expect(lines.length).toBeGreaterThan(0);
+        lines.forEach((line) => {
+            expect(line.getAttribute("data-dot")).toBe("false");
+            expect(line.getAttribute("data-stroke-width")).toBe("1");
+        });
+    });
+
+    it("pads the y domain via a hidden axis so edge-hugging series are not clipped", () => {
+        const { getByTestId } = render(<ChartCell {...mkProps(validChart)} />);
+        const axis = getByTestId("yaxis-mock");
+        expect(axis.getAttribute("data-hide")).toBe("true");
+        expect(axis.getAttribute("data-padding-top")).toBe("2");
+        expect(axis.getAttribute("data-padding-bottom")).toBe("2");
+    });
+});
diff --git a/packages/buckaroo-js-core/src/components/DFViewerParts/HistogramCell.colors.test.tsx b/packages/buckaroo-js-core/src/components/DFViewerParts/HistogramCell.colors.test.tsx
new file mode 100644
index 000000000..a367434f7
--- /dev/null
+++ b/packages/buckaroo-js-core/src/components/DFViewerParts/HistogramCell.colors.test.tsx
@@ -0,0 +1,79 @@
+/**
+ * Per-bar color support on the histogram cell: a HistogramBar may carry
+ * `color`, and the population Bar renders one recharts Cell per datum so
+ * the color lands on that individual bar. Diff views use this to paint the
+ * change-distribution histogram with the same color key as the data cells;
+ * bars without a color keep the scheme default.
+ */
+// jsdom's `crypto` lacks `randomUUID`; HistogramCell's gensym() uses it.
+{
+    let n = 0;
+    const existing: any = (globalThis as any).crypto || {};
+    if (typeof existing.randomUUID !== "function") {
+        try {
+            Object.defineProperty(existing, "randomUUID", {
+                configurable: true,
+                value: () => `test-uuid-${++n}`,
+            });
+        } catch {
+            Object.defineProperty(globalThis, "crypto", {
+                configurable: true,
+                value: { ...existing, randomUUID: () => `test-uuid-${++n}` },
+            });
+        }
+    }
+}
+
+// ts-jest in this repo doesn't apply esModuleInterop, so the default import
+// `import React from "react"` in HistogramCell.tsx resolves to `undefined`
+// at runtime without a `.default` on the mock.
+jest.mock("react", () => {
+    const actual = jest.requireActual("react");
+    return { __esModule: true, default: actual, ...actual };
+});
+
+import { render } from "@testing-library/react";
+import type { ColDef, Column, Context, GridApi } from "ag-grid-community";
+import { HistogramCell } from "./HistogramCell";
+
+// recharts pulls in DOM-measurement code that doesn't run cleanly under
+// jsdom — stub the exports HistogramCell touches, keeping Bar/Cell props
+// inspectable in the rendered tree.
+jest.mock("recharts", () => {
+    const React = require("react");
+    return {
+        Bar: ({ children, dataKey }: any) =>
+            React.createElement("div", { "data-testid": `bar-${dataKey}` }, children),
+        BarChart: ({ children }: any) =>
+            React.createElement("div", { "data-testid": "barchart-mock" }, children),
+        Cell: ({ fill }: any) =>
+            React.createElement("div", { "data-testid": "cell-mock", "data-fill": fill }),
+        Tooltip: () => null,
+    };
+});
+
+const mkProps = (value: any) => ({
+    value,
+    api: {} as GridApi,
+    colDef: { cellClass: "" } as ColDef,
+    column: {} as Column,
+    context: {} as Context,
+});
+
+describe("HistogramCell per-bar colors", () => {
+    it("renders one Cell per datum on the population bar, honoring bar.color", () => {
+        const bars = [
+            { name: "<-50%", population: 20, color: "#d62728" },
+            { name: "~0%", population: 80 },
+        ];
+        const { getByTestId } = render(<HistogramCell {...mkProps(bars)} />);
+        const popBar = getByTestId("bar-population");
+        const cells = popBar.querySelectorAll('[data-testid="cell-mock"]');
+        expect(cells).toHaveLength(2);
+        expect(cells[0].getAttribute("data-fill")).toBe("#d62728");
+        // Uncolored bars keep the scheme default fill — set, and not the
+        // colored bar's value.
+        expect(cells[1].getAttribute("data-fill")).toBeTruthy();
+        expect(cells[1].getAttribute("data-fill")).not.toBe("#d62728");
+    });
+});
diff --git a/tests/unit/compare_test.py b/tests/unit/compare_test.py
index 33d6136bd..e912e060b 100644
--- a/tests/unit/compare_test.py
+++ b/tests/unit/compare_test.py
@@ -464,3 +464,20 @@ def test_merge_column_in_input_rejected():
 
     with pytest.raises(ValueError, match="__buckaroo_merge"):
         col_join_dfs(df1, df2, join_columns=["id"], how="outer")
+
+
+def test_join_key_columns_use_color_static():
+    """Join-key columns get the constant color_static rule.
+
+    The categorical-map-of-identical-colors workaround predates color_static
+    landing in the compiled JS; the overrides should now emit the rule
+    directly so the Python types and the wire config say what they mean.
+    """
+    df1 = pd.DataFrame({"id": [1, 2, 3], "val": [10, 20, 30]})
+    df2 = pd.DataFrame({"id": [1, 2, 3], "val": [10, 25, 30]})
+
+    _m_df, overrides, _eqs = col_join_dfs(df1, df2, join_columns=["id"], how="outer")
+
+    cfg = overrides["id"]["color_map_config"]
+    assert cfg["color_rule"] == "color_static"
+    assert cfg["color"]