diff --git a/plugins/plotly-express/src/js/src/PlotlyExpressChartModel.test.ts b/plugins/plotly-express/src/js/src/PlotlyExpressChartModel.test.ts
new file mode 100644
index 000000000..57ef108b5
--- /dev/null
+++ b/plugins/plotly-express/src/js/src/PlotlyExpressChartModel.test.ts
@@ -0,0 +1,195 @@
+import { dh as DhType } from '@deephaven/jsapi-types';
+import { TestUtils } from '@deephaven/utils';
+import { PlotlyExpressChartModel } from './PlotlyExpressChartModel';
+import { PlotlyChartWidgetData } from './PlotlyExpressChartUtils';
+
+jest.mock('./PlotlyExpressChartModel', () => {
+  const original = jest.requireActual('./PlotlyExpressChartModel');
+  original.PlotlyExpressChartModel.prototype.fireDownsampleStart = jest.fn();
+  original.PlotlyExpressChartModel.prototype.fireDownsampleFinish = jest.fn();
+  original.PlotlyExpressChartModel.prototype.fireDownsampleFail = jest.fn();
+  original.PlotlyExpressChartModel.prototype.fireDownsampleNeeded = jest.fn();
+  return original;
+});
+
+const SMALL_TABLE = TestUtils.createMockProxy<DhType.Table>({
+  columns: [{ name: 'x' }, { name: 'y' }] as DhType.Column[],
+  size: 500,
+  subscribe: () => TestUtils.createMockProxy(),
+});
+
+const LARGE_TABLE = TestUtils.createMockProxy<DhType.Table>({
+  columns: [{ name: 'x' }, { name: 'y' }] as DhType.Column[],
+  size: 50000,
+  subscribe: () => TestUtils.createMockProxy(),
+});
+
+function createMockWidget(tables: DhType.Table[], plotType = 'scatter') {
+  const widgetData = {
+    type: 'test',
+    figure: {
+      deephaven: {
+        mappings: tables.map((_, i) => ({
+          table: i,
+          data_columns: {
+            x: [`/plotly/data/${i}/x`],
+            y: [`/plotly/data/${i}/y`],
+          },
+        })),
+        is_user_set_color: false,
+        is_user_set_template: false,
+      },
+      plotly: {
+        data: tables.map((_, i) => ({
+          type: plotType as 'scatter',
+          mode: 'lines',
+          xaxis: i === 0 ? 'x' : `x${i + 1}`,
+          yaxis: i === 0 ? 'y' : `y${i + 1}`,
+        })),
+        layout: {
+          title: 'layout',
+          xaxis: {},
+          yaxis: {},
+          xaxis2: {},
+          yaxis2: {},
+        },
+      },
+    },
+    revision: 0,
+    new_references: tables.map((_, i) => i),
+    removed_references: [],
+  } satisfies PlotlyChartWidgetData;
+
+  return {
+    getDataAsString: () => JSON.stringify(widgetData),
+    exportedObjects: tables.map(t => ({
+      fetch: () => Promise.resolve(t),
+      reexport: jest.fn(),
+      close: jest.fn(),
+    })),
+    addEventListener: jest.fn(),
+  } satisfies Partial<DhType.Widget> as unknown as DhType.Widget;
+}
+
+type DeepPartial<T> = T extends object
+  ? {
+      [P in keyof T]?: DeepPartial<T[P]>;
+    }
+  : T;
+
+const mockDownsample = jest.fn(t => t);
+
+const mockDh = {
+  calendar: {
+    DayOfWeek: {
+      values: () => [],
+    },
+  },
+  plot: {
+    Downsample: {
+      runChartDownsample: mockDownsample,
+    },
+    ChartData: (() =>
+      TestUtils.createMockProxy()) as unknown as typeof DhType.plot.ChartData,
+  },
+  Table: {
+    EVENT_UPDATED: 'updated',
+  },
+  Widget: {
+    EVENT_MESSAGE: 'message',
+  },
+} satisfies DeepPartial<typeof DhType> as unknown as typeof DhType;
+
+beforeEach(() => {
+  jest.resetAllMocks();
+});
+
+describe('PlotlyExpressChartModel', () => {
+  it('should create a new instance of PlotlyExpressChartModel', () => {
+    const mockWidget = createMockWidget([]);
+
+    const chartModel = new PlotlyExpressChartModel(
+      mockDh,
+      mockWidget,
+      jest.fn()
+    );
+
+    expect(chartModel.isSubscribed).toBe(false);
+    expect(chartModel.layout).toEqual(
+      JSON.parse(mockWidget.getDataAsString()).figure.plotly.layout
+    );
+  });
+
+  it('should subscribe', async () => {
+    const mockWidget = createMockWidget([]);
+    const chartModel = new PlotlyExpressChartModel(
+      mockDh,
+      mockWidget,
+      jest.fn()
+    );
+
+    await chartModel.subscribe(jest.fn());
+    expect(chartModel.isSubscribed).toBe(true);
+  });
+
+  it('should not downsample line charts when the table is small', async () => {
+    const mockWidget = createMockWidget([SMALL_TABLE]);
+    const chartModel = new PlotlyExpressChartModel(
+      mockDh,
+      mockWidget,
+      jest.fn()
+    );
+
+    await chartModel.subscribe(jest.fn());
+    expect(mockDownsample).toHaveBeenCalledTimes(0);
+    await new Promise(process.nextTick); // Subscribe and addTable are async
+    expect(chartModel.fireDownsampleStart).toHaveBeenCalledTimes(0);
+    expect(chartModel.fireDownsampleFinish).toHaveBeenCalledTimes(0);
+  });
+
+  it('should downsample line charts when the table is big', async () => {
+    const mockWidget = createMockWidget([LARGE_TABLE]);
+    const chartModel = new PlotlyExpressChartModel(
+      mockDh,
+      mockWidget,
+      jest.fn()
+    );
+
+    await chartModel.subscribe(jest.fn());
+    expect(mockDownsample).toHaveBeenCalledTimes(1);
+    await new Promise(process.nextTick); // Subscribe and addTable are async
+    expect(chartModel.fireDownsampleStart).toHaveBeenCalledTimes(1);
+    expect(chartModel.fireDownsampleFinish).toHaveBeenCalledTimes(1);
+  });
+
+  it('should downsample only the required tables', async () => {
+    const mockWidget = createMockWidget([SMALL_TABLE, LARGE_TABLE]);
+    const chartModel = new PlotlyExpressChartModel(
+      mockDh,
+      mockWidget,
+      jest.fn()
+    );
+
+    await chartModel.subscribe(jest.fn());
+    expect(mockDownsample).toHaveBeenCalledTimes(1);
+    await new Promise(process.nextTick); // Subscribe and addTable are async
+    expect(chartModel.fireDownsampleStart).toHaveBeenCalledTimes(1);
+    expect(chartModel.fireDownsampleFinish).toHaveBeenCalledTimes(1);
+  });
+
+  it('should fail to downsample for non-line plots', async () => {
+    const mockWidget = createMockWidget([LARGE_TABLE], 'scatterpolar');
+    const chartModel = new PlotlyExpressChartModel(
+      mockDh,
+      mockWidget,
+      jest.fn()
+    );
+
+    await chartModel.subscribe(jest.fn());
+    expect(mockDownsample).toHaveBeenCalledTimes(0);
+    await new Promise(process.nextTick); // Subscribe and addTable are async
+    expect(chartModel.fireDownsampleStart).toHaveBeenCalledTimes(1);
+    expect(chartModel.fireDownsampleFinish).toHaveBeenCalledTimes(0);
+    expect(chartModel.fireDownsampleFail).toHaveBeenCalledTimes(1);
+  });
+});
diff --git a/plugins/plotly-express/src/js/src/PlotlyExpressChartModel.ts b/plugins/plotly-express/src/js/src/PlotlyExpressChartModel.ts
index f6a7b0ca3..0cbfb0af8 100644
--- a/plugins/plotly-express/src/js/src/PlotlyExpressChartModel.ts
+++ b/plugins/plotly-express/src/js/src/PlotlyExpressChartModel.ts
@@ -14,7 +14,7 @@ import {
   isLineSeries,
   isLinearAxis,
   removeColorsFromData,
-} from './PlotlyExpressChartUtils.js';
+} from './PlotlyExpressChartUtils';
 
 const AUTO_DOWNSAMPLE_SIZE = 30000;