Voronoi Mark

voronoi partitions the final plot into the cells nearest to each unique scaled x/y position. It emits renderer-neutral polygons clipped to the final plot bounds. Import it from the optional spatial subpath; it is not included in the root or universal barrel.

import { dot } from '@tanstack/charts'
import { voronoi } from '@tanstack/charts/spatial/voronoi'

const marks = [
  voronoi(cars, {
    x: 'weight',
    y: 'economy',
    key: 'id',
    color: 'cylinders',
    fillOpacity: 0.14,
    stroke: '#fff',
    strokeWidth: 1,
  }),
  dot(cars, {
    x: 'weight',
    y: 'economy',
    key: 'id',
    color: 'cylinders',
  }),
]

function voronoi<TDatum>(
  source: Iterable<TDatum>,
  options: VoronoiOptions<TDatum>,
): ChartMark<never, never, never, InferredX, InferredY>

The mark contributes x/y and color domains but intentionally emits no ChartPoint interaction candidates.

Options

fill and stroke accessors receive the original source row, source index, and complete source array.

Final-screen topology

Complete x/y pairs establish the positional domains. After scales and inner bounds resolve, the mark projects both axes, computes the Voronoi diagram in screen space, and clips every cell to the plot rectangle. Resizing can change cell geometry even when source values and domains are unchanged.

Inversion is not required, so continuous, temporal, and categorical positional scales are supported. Missing, nonfinite, or unmappable pairs are omitted without allowing one coordinate to expand the other coordinate's domain. Zero valid positions produce no cells; one unique position fills the plot; two positions split it; and collinear positions remain valid.

The generated polygons are ordinary scene areas, not SVG path strings, so the same definition works with SVG, Canvas, and static rendering.

Groups and identity

An explicit z creates an independent full-plot tessellation for every group. Those diagrams overlap. Omit z when all observations should share one partition; use color alone to paint categories without changing topology.

Supply key when source order can change. Exact coincident positions produce one cell within each z group, with ownership chosen from a canonical ordering of stable keys. This makes coincident ownership and cell keys deterministic across reorderings. Distinct projected positions remain distinct even when they are much closer than one pixel because nearby sites can still divide a large visible region. The mark fails instead of merging sites if separate cell boundaries cannot be represented by finite screen coordinates. Without an explicit key, the mark tries a unique top-level or nested id, then falls back to source index.

Visible cells versus focus

Voronoi polygons are a visible encoding layer, not an interaction index. They are hidden from accessibility navigation and add no pointer or keyboard focus candidates. Layer dot or another interactive mark over the cells when the source observations should drive nearest-point focus and tooltips. Both layers can share x, y, color, and key channels without application-owned scale projection or tooltip state.

If no cells should be painted and only lookup performance matters, provide a ChartSpatialIndexFactory instead of adding a voronoi mark. See Tooltips and Focus.