Grid mark doesn’t use its corresponding axis’ ticks

[mwaibel-go]

The documentation for the grid mark states:

The optional data is an array of tick values — it defaults to the scale’s ticks.

The following example reproduces a case in which the grid mark ignores the scale’s ticks and instead produces its own:

Plot.plot({
  x: {
    domain: [new Date(2026, 2, 9).valueOf(), new Date(2026, 2, 11).valueOf()],
    ticks: ['2026-03-09','2026-03-10','2026-03-11']
      .map(d => new Date(d).valueOf()),
  },
  marks: [
    Plot.gridX(),
  ],
})

[mbostock]

The documentation is referring to the implicit grid mark (the “default grid”) that is created when you set grid: true on a scale, rather than the default ticks when you create an explicit grid mark. Here is the recommended pattern using an implicit grid (and axis) mark:

Plot.plot({
  x: {
    domain: [new Date(2026, 2, 9).valueOf(), new Date(2026, 2, 11).valueOf()],
    ticks: ["2026-03-09", "2026-03-10", "2026-03-11"].map((d) => new Date(d).valueOf()),
    grid: true
  }
})

If you create an explicit grid mark (or axis mark), you should specify the data (or equivalently the ticks option) on the mark directly, rather than as a scale option. For example:

Plot.plot({
  x: {
    domain: [new Date(2026, 2, 9).valueOf(), new Date(2026, 2, 11).valueOf()]
  },
  marks: [
    ((ticks) => [
      Plot.gridX(ticks),
      Plot.axisX(ticks)
    ])(["2026-03-09", "2026-03-10", "2026-03-11"].map((d) => new Date(d).valueOf()))
  ]
})