ChoroplethChart

A headless wrapper for a map chart that shades regions by the value of a measure. It gives the drawing the catalog's standard accessible shell: a named image plus an optional data-table alternative.

Status: beta — implemented and unit-tested in all eight headless libraries; not yet exercised in composed flows.

The component renders a <figure> holding a <div class="choropleth-chart-graphic" role="img" aria-label> around an inline <svg> that the consumer draws, and an optional sibling <div class="choropleth-chart-data-table"> for the accessible table. The component draws nothing and ships no scales, colours or animation.

Implementation Notes

• Renders <figure class="choropleth-chart {class}"> containing the image wrapper and, when supplied, the data-table wrapper
• The consumer supplies the <svg> (and any legend markup that belongs inside the image)
• restProps spread onto the <figure>
• No internal state, no drawing, no data handling

Props

Usage

<ChoroplethChart label="Describe the chart">
  <svg viewBox="0 0 100 100">…</svg>
  {#snippet dataTable()}
    <table><caption>Values</caption>…</table>
  {/snippet}
</ChoroplethChart>

Keyboard Interactions

• None on the graphic. The chart is a single image to assistive technology.
• The data table follows native table behaviour.

ARIA

• role="img" on the graphic wrapper exposes the chart as one image (its children are presentational)
• aria-label provides the accessible name
• The data table is outside role="img" so assistive technology can read it

When to Use

• A measure compared across regions such as countries, counties or postcodes
• Showing geographic patterns at a glance
• When a real data table accompanies the drawing as the accessible alternative

When Not to Use

• Use TileMap — a map with pins or a map of places rather than shaded regions
• Use BarChart — when exact comparison between regions matters more than geography
• Use a table — when the user needs to look up a specific region

Headless

This component decides semantics only: the figure element, the image role, the name and the table placement. It decides no geometry, scale, colour, legend or motion.

Styles

Target .choropleth-chart for the figure, .choropleth-chart-graphic for the image wrapper and .choropleth-chart-data-table for the table wrapper. Style the supplied <svg> from consumer CSS. No default styles are included.

Testing

• Renders a <figure> with class choropleth-chart and a .choropleth-chart-graphic child with role="img"
• label sets aria-label
• The consumer svg renders inside the image wrapper
• The data table renders in a .choropleth-chart-data-table sibling, outside role="img", only when supplied
• Rest props reach the figure

Advice

Pass the shaded regions as <svg> paths. Region size misleads (large, sparse regions dominate), so give every region and its value in the data table and never rely on colour alone.

Because role="img" makes descendants presentational, never put interactive controls inside the graphic; place legends and toggles next to it.

Data table alternative (added 2026-10-05)

The graphic is wrapped in <div class="choropleth-chart-graphic" role="img" aria-label>; role="img" is not on the <figure>. The optional dataTable (a snippet in Svelte, a prop in React, a named slot in Vue, a projected [dataTable] element in Angular, a DataTable render fragment in Blazor, params.dataTable in Nunjucks, a slot="data-table" child in Web Components) renders the accessible table in <div class="choropleth-chart-data-table">, a sibling of the graphic and never inside it: role="img" makes its descendants presentational, so a table inside would be invisible to assistive technology. Without a data table the wrapper is not rendered (except Angular, which always renders an empty wrapper because it cannot detect projected content).

Related components

• tile-map
• bar-chart
• heatmap-chart
• graphic-block

References

• MDN figure element
• W3C WAI: Complex images

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