Extend TanStack Charts

Use trigger → inspect → decide → build → verify. Extend the narrowest ownership boundary after proving built-in marks, first-party layouts, transforms, facets, views, and controlled behaviors cannot express the required semantics.

Setup

Create a renderer-neutral mark with declared scale values and deterministic scene keys:

import { createMark, defineChart } from '@tanstack/charts'
import { scaleLinear } from '@tanstack/charts/scales/linear'

interface ThresholdDatum {
  id: string
  value: number
}

const threshold = createMark<ThresholdDatum, never, number>(({ markIndex }) => {
  const datum: ThresholdDatum = { id: 'target', value: 75 }