Filter render transform

[mbostock]

Here’s a sketch on a more imperative, lower-level interaction primitive: a filter render transform that you can dynamically update whenever you like. It starts by declaring some filters (and their initial filter state):

const pointerInactive = renderFilter(true);
const pointerContext = renderFilter(false);
const pointerFocus = renderFilter(false);

Then you apply the filters to the desired marks like any other transform:

const plot = Plot.plot({
  y: {grid: true},
  color: {scheme: "BuRd", symmetric: false},
  marks: [
    Plot.ruleY([0]),
    Plot.lineY(bls, pointerInactive({x: "date", y: "unemployment", z: "division", tip: true})),
    Plot.lineY(bls, pointerContext({x: "date", y: "unemployment", z: "division", stroke: "#ccc"})),
    Plot.lineY(bls, pointerFocus({x: "date", y: "unemployment", z: "division", stroke: "red"}))
  ]
});

Lastly here I use an event listener (here driven by the tip mark of the plot itself!) to update the filters:

plot.addEventListener("input", () => {
  if (plot.value === null) {
    pointerInactive.update(true);
    pointerContext.update(false);
    pointerFocus.update(false);
  } else {
    const division = plot.value.division;
    pointerInactive.update(false);
    pointerContext.update((d) => d.division !== division);
    pointerFocus.update((d) => d.division === division);
  }
});

https://github.com/observablehq/plot/assets/230541/bfbb8274-05e9-4945-b22d-87ea0ba7b302

TODO

• [ ] Support faceting
• [ ] Incorporate into Plot as a public API
• [ ] Document
• [ ] Test?

Fixes #1857. Related #1574.

[Fil]

Exciting!

To support faceting we only need to re-apply the translate attribute (we can copy it from the old g). https://observablehq.com/@observablehq/filtering-several-charts-1871

I'd like to also support sharing the behavior between charts, but it's a bit more delicate (it needs an invalidation strategy).