@vit-foundation/chart

Charts, the scales behind them, and the legends that describe them.

Two ideas hold the package together.

A renderer that paints classed data and a legend that explains it must agree on what a class is. classColors / classOpacities are that agreement for the colour channel, and weightRange is the same agreement for a thickness channel. Hand both sides the same function and they cannot drift.

A chart's geometry and its appearance are different things. A chart resolves its box into a plot frame — margins, scales, tick counts, which labels fit — and a mark decides what the series looks like in it. Adding a visual style is one small component, never a branch inside a chart.

Extracted from the National Geographic Food for Tomorrow explore globe, where the same arithmetic already had four readers — a GPU renderer's stepped visual variables, a CSS hover mirror, the legend bar and the tier dots — and drift between them was the standing hazard.

It knows nothing about maps and nothing about any dataset. Two dependencies (chroma-js, d3-scale) and one peer (svelte, for the components).

Install

npm install @vit-foundation/chart

A chart

<script>
	import SeriesPlot from '@vit-foundation/chart/SeriesPlot.svelte';

	const rows = [
		{ x: 2000, y: 326121011, weight: 4804500 },
		{ x: 2001, y: 344292922, weight: 4957900 }
		// …
	];
</script>

<div style="height: 50vh">
	<SeriesPlot
		{rows}
		mark="ribbon"
		markStyle={{ color: '#FFD700', outlineColor: '#d9b700', outlineWidth: 2 }}
		yLabel="↑ million tonnes"
		xLabel="year →"
		legend={{ title: 'Area harvested' }}
	/>
</div>

Rows are { x, y, weight } — deliberately not a dataset's column names. The chart draws what it is given and fetches nothing, so the same component serves a CSV, an API and a fixture.

Marks

mark takes a name from the built-in table or a component of your own:

name	draws
ribbon	a curve whose thickness follows weight
line	a plain constant-width curve
area	a filled area down to the baseline

A mark is an ordinary Svelte component handed { frame, rows, extents, style }. It renders SVG into the plotting area and derives nothing: the frame already resolved every measurement. Nothing in the package enumerates the marks, so a new style never edits a file you do not own:

<!-- Dots.svelte -->
<script>
	let { frame, rows, style } = $props();
</script>

{#each rows as r (r.x)}
	<circle cx={frame.x(r.x)} cy={frame.y(r.y)} r="3" fill={style.color} />
{/each}

<SeriesPlot {rows} mark={Dots} />

Hover

The chart owns hit-testing, because it is the only thing that knows which row a pointer is over. It does not own the card — onhover reports the row and the point, and the host draws whatever its design system draws.

<SeriesPlot {rows} onhover={(row, point) => (tip = { row, point })} onleave={() => (tip = null)} />

The scale

import { classColors, classOpacities } from '@vit-foundation/chart/scale';

const k = breaks.length + 1;
classColors(['#fff7bc', '#fec44f', '#d95f0e'], k); // one hex per class, light→dark
classOpacities(k, 0.2, 1); // one alpha per class, low→high

Hand the same ramp and the same k to whatever paints the data, and the legend and the rendering cannot disagree.

The legend

<script>
	import PercentLegend from '@vit-foundation/chart/PercentLegend.svelte';
</script>

<PercentLegend colors={RAMP} max={72} breaks={[6.6, 18.7, 35.6, 58.4]} />

breaks classes the colour channel; opacity classes (or ramps) the alpha channel; the two composite independently, so mismatched break sets still render correctly. flat is the opacity-only variant — the bar holds the darkest ramp colour and lets alpha carry the value. mode="ticks" draws break marks instead of hard-edged bands.

The arithmetic is exported separately, so you can build your own bar:

import { bandEdges, barGradient, pickLabels, ticksOf } from '@vit-foundation/chart/legend';

Theming

The component ships plain CSS with custom-property hooks, so it renders standalone and restyles without a CSS framework:

property	default
--vit-legend-font	inherit
--vit-legend-label-color	currentColor
--vit-legend-label-size	0.75rem
--vit-legend-tick-size	0.625rem
--vit-legend-stroke	#424c5c
--vit-legend-bar-height	0.375rem
--vit-legend-max-label-width	3.5rem

The chart components read their own set:

property	default
--vit-chart-font	inherit
--vit-chart-axis-color	currentColor
--vit-chart-grid-color	#cacaca
--vit-chart-grid-opacity	0.5
--vit-chart-hatch-color	#00000020
--vit-chart-highlight-ring	#ffffff

Subpaths

Import the narrow subpath, not the barrel:

subpath	holds
@vit-foundation/chart/plot	the plot frame + axis rules, no DOM
@vit-foundation/chart/series	the row contract and extents, no DOM
@vit-foundation/chart/marks	the mark contract, registry and kernel
@vit-foundation/chart/scale	classColors, classOpacities
@vit-foundation/chart/legend	the bar arithmetic, no Svelte, no DOM
@vit-foundation/chart/SeriesPlot.svelte	the chart
@vit-foundation/chart/PercentLegend.svelte	the colour-scale legend
@vit-foundation/chart/WeightWedge.svelte	the thickness key
@vit-foundation/chart/PlotAxes.svelte	grid, ticks and axis labels

The two .ts subpaths run in plain Node. The barrel (@vit-foundation/chart) re-exports the component, so it needs a Svelte-aware bundler — reach for it only from an app that has one.

Scope

In: charts built on the plot-frame + mark seam, colour-scale arithmetic, and the legends that depict either.

Out: anything that describes a symbol rather than a scale or a mark (size rings, alpha tiers — those are map vocabulary), and anything that knows where data came from. Loading, parsing and a dataset's column names stay in the app.

This section used to read "a chart you built for one story is not a library", which was the right rule for a chart with no seam in it: generalising one story's graphic by adding props gives you a component with thirty of them and one caller. The line moved on 2026-09-21 because the mark seam changes the economics — the package holds the frame and the contract, and a style is a component anyone can write without touching this repo.

Documentation

The full site lives in docs/ and is served by npm run dev:docs:

• Getting started — install, the subpaths, a legend on screen
• The scale — class bands, and the contract they hold
• The legend — the two channels, the modes, theming
• Reference — every entry point and what it exports
• Changelog — one page per version, newest first

CONTRIBUTING has the setup, the rules the tooling enforces, and the release ritual.

Package name

This package was published as @vit-foundation/vizvit-chart up to 0.0.3, which contained only the ExampleChart scaffold. It is @vit-foundation/chart from 0.1.0. ExampleChart is still exported from the barrel; nothing was removed.

License

Apache-2.0