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testland/chart-render-tests

Use when a dashboard renders charts that need regression tests, before a chart library upgrade or after a theme change. Canvas charts (Chart.js) get screenshot and toDataURL diffs, SVG (D3) gets normalized structure snapshots, and Vega or Vega-Lite specs get schema and compile checks.

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Vega/Vega-Lite advanced spec tests

Deep reference for the Vega side of chart-render-tests (vega.md). Consult after the core schema + structure + compilation gates pass, when adding render-time, composition, transform, interaction, or spec-snapshot coverage.

Render-to-SVG test

import { Vega } from 'react-vega';
import { create } from 'jsdom';

test('renders SVG with expected mark count', async () => {
  const dom = create('<div id="vis"></div>');
  global.document = dom.window.document;

  const view = new vega.View(vega.parse(vegaSpec))
    .renderer('svg')
    .initialize(dom.window.document.querySelector('#vis'))
    .run();

  const svg = await view.toSVG();
  // Use a parser to count <path>/<rect> elements
  const rectCount = (svg.match(/<rect/g) || []).length;
  expect(rectCount).toBe(4);  // 4 quarters
});

Multi-view composition test

Per the Vega-Lite docs, Vega-Lite supports faceting, layering, concatenation, repeating. Test each composition:

test('layered spec has 2 layers', () => {
  const spec = {
    layer: [
      { mark: 'line', encoding: {...} },
      { mark: 'point', encoding: {...} },
    ],
  };

  expect(validate(spec)).toBe(true);
  expect(spec.layer).toHaveLength(2);
});

test('faceted spec creates one view per category', async () => {
  const spec = {
    facet: { field: 'category', type: 'nominal' },
    spec: { mark: 'bar', encoding: {...} },
  };

  const compiled = vl.compile(spec).spec;
  const view = new vega.View(vega.parse(compiled)).renderer('svg').initialize(...).run();
  // Inspect view's data tables to verify N facets emerged
});

Data transform test

Per the Vega-Lite docs, transforms include "Aggregate, filter, bin, calculate, fold, pivot." Test transform output:

test('aggregate transform produces correct sum', () => {
  const spec = {
    data: { values: [
      { region: 'NA', revenue: 100 },
      { region: 'NA', revenue: 200 },
      { region: 'EU', revenue: 150 },
    ]},
    transform: [
      { aggregate: [{ op: 'sum', field: 'revenue', as: 'total' }],
        groupby: ['region'] },
    ],
    mark: 'bar',
    encoding: { x: { field: 'region' }, y: { field: 'total', type: 'quantitative' } },
  };

  const view = new vega.View(vega.parse(vl.compile(spec).spec));
  await view.runAsync();
  const data = view.data('source_0');
  expect(data.find(d => d.region === 'NA').total).toBe(300);
  expect(data.find(d => d.region === 'EU').total).toBe(150);
});

Interaction (parameters / selections)

Per the Vega-Lite docs, "Interactive parameters - Selections and value bindings" enable interaction. Test parameters resolve:

test('selection parameter filters data', async () => {
  const spec = {
    params: [{ name: 'brush', select: 'interval' }],
    data: {...},
    mark: 'point',
    encoding: {...},
    transform: [{ filter: { param: 'brush' } }],
  };

  // Compile, render, inject brush event, verify filtered data
  ...
});

Spec-snapshot regression

For complex spec-generation logic, snapshot the spec output:

test('quarterly-revenue spec snapshot stable', () => {
  const spec = generateBarSpec({
    x: 'quarter',
    y: 'revenue',
    title: 'Quarterly Revenue',
  });

  expect(spec).toMatchSnapshot('quarterly-revenue.spec.json');
});

When intentionally changing spec generation, regenerate snapshots: UPDATE_SNAPSHOTS=1 npm test.