chartjs-snapshot-tests

Snapshot-test Chart.js charts - render via headless Chromium / jsdom + canvas mock, capture canvas pixels via `canvas.toDataURL()` + image-diff, disable animations (`options.animation = false`) for stable snapshots, test tooltip + legend interactions. Pairs with qa-visual-regression for general UI snapshot patterns.

chartjs-snapshot-tests

Per the Chart.js docs, Chart.js renders to <canvas>. Canvas output is a pixel buffer - testable via canvas.toDataURL() snapshot diff.

When to use

Dashboards or analytics products where chart accuracy is product surface.
Library upgrade gate (Chart.js v4 → v5 changes default styles).
Custom theme integration - verify the brand styling renders correctly.

Step 1 - Disable animations for stable snapshots

Per the Chart.js docs, the basic config object accepts options:

new Chart(ctx, {
  type: 'bar',
  data: {...},
  options: {
    animation: false,    // disable for snapshots
    responsive: false,   // fix the canvas dimensions
    plugins: {
      legend: { display: true },
    },
    scales: { y: { beginAtZero: true } },
  },
});

Without animation: false, snapshots capture mid-animation frames randomly.

Step 2 - Playwright canvas snapshot

import { test, expect } from '@playwright/test';

test('revenue bar chart matches snapshot', async ({ page }) => {
  await page.goto('https://localhost:3000/dashboard');

  // Wait for chart to render (no animation, just initial draw)
  await page.waitForFunction(() => {
    const canvas = document.querySelector('canvas#revenue-chart');
    return canvas && canvas.toDataURL().length > 1000;
  });

  const canvas = page.locator('canvas#revenue-chart');
  await expect(canvas).toHaveScreenshot('revenue-chart.png', {
    maxDiffPixels: 50,
  });
});

maxDiffPixels allows for sub-pixel anti-aliasing variance across runs.

Step 3 - Programmatic canvas dataURL diff

For finer control without Playwright's screenshot helper:

test('chart canvas data URL is stable', async ({ page }) => {
  await page.goto('https://localhost:3000/dashboard');
  await page.waitForFunction(() => /* render complete */);

  const dataUrl = await page.evaluate(() => {
    const canvas = document.querySelector('canvas#revenue-chart') as HTMLCanvasElement;
    return canvas.toDataURL('image/png');
  });

  // Compare to baseline saved as PNG
  const baseline = await readBaseline('revenue-chart.png');
  const diff = imagePixelDiff(dataUrl, baseline);
  expect(diff.diffRatio).toBeLessThan(0.005);
});

Step 4 - jsdom + canvas-mock unit testing

For unit-test-speed feedback (no browser):

// jest.setup.js
import 'canvas';  // node-canvas package

import { Chart } from 'chart.js/auto';

test('chart renders with expected dataset count', () => {
  const canvas = document.createElement('canvas');
  document.body.appendChild(canvas);

  const chart = new Chart(canvas, {
    type: 'bar',
    data: { labels: ['Q1', 'Q2'], datasets: [{ data: [10, 20] }] },
    options: { animation: false, responsive: false },
  });

  expect(chart.data.datasets).toHaveLength(1);
  expect(chart.data.labels).toEqual(['Q1', 'Q2']);
});

canvas package (Node native) lets jsdom render Chart.js output without a browser. Use for fast assertions on dataset shape + config; rely on Step 2 for visual regression.

Step 5 - Tooltip + legend interaction

test('tooltip shows data point value on hover', async ({ page }) => {
  await page.goto('https://localhost:3000/dashboard');
  await waitForChartReady(page);

  // Hover over a known data point coordinate
  await page.mouse.move(150, 200);
  await page.waitForSelector('.chartjs-tooltip', { state: 'visible' });

  const tooltipText = await page.locator('.chartjs-tooltip').textContent();
  expect(tooltipText).toContain('Q1: 10');
});

test('legend click toggles dataset visibility', async ({ page }) => {
  await page.goto('https://localhost:3000/dashboard');
  await waitForChartReady(page);

  await page.click('.chartjs-legend-item:has-text("Revenue")');

  // Re-snapshot; revenue dataset should be hidden
  await expect(page.locator('canvas#revenue-chart')).toHaveScreenshot(
    'revenue-chart-revenue-hidden.png'
  );
});

Step 6 - Data-driven assertion (without snapshot)

For non-visual assertions, query Chart.js internal state via the chart instance:

test('chart shows all 12 months', async ({ page }) => {
  const labels = await page.evaluate(() => {
    const chart = (window as any).Chart.getChart('revenue-chart');
    return chart.data.labels;
  });
  expect(labels).toHaveLength(12);
});

Step 7 - Multi-DPI handling

High-DPI displays render canvas at 2× / 3× device pixel ratio. Snapshots taken at different DPRs differ. Pin DPR in test config:

// playwright.config.ts
use: {
  deviceScaleFactor: 1,  // pin to 1× for snapshot stability
}

Anti-patterns

Anti-pattern	Why it fails	Fix
Skip `animation: false`	Snapshots flaky	Step 1 mandatory
Snapshot whole page	Layout shifts unrelated to chart break tests	Snapshot the canvas locator only (Step 2)
`maxDiffPixels: 0`	Anti-aliasing flake	Allow ~50 pixels (Step 2)
Test only static data	Dynamic data behavior untested	Snapshot per scenario (filter, range)
Skip DPR pinning	CI machines vs dev machines render differently	Step 7

Limitations

Canvas snapshots can't catch SVG-only regressions (Chart.js is canvas-only); for SVG charts use d3-snapshot-tests.
Chart.js plugins (annotation, datalabels) may have separate init paths; verify they render before snapshotting.
Tooltips render in DOM (not canvas), so canvas snapshot misses them - test interactions separately (Step 5).

References

Chart.js docs - install, basic config, options
d3-snapshot-tests - sister skill for SVG-based charts
vega-spec-validator - sister skill for declarative-spec validation
node-canvas - github.com/Automattic/node-canvas (Node-native Canvas implementation for unit tests)