The map transform should maybe apply maybeApplyInterval?

The map transform groups by y and not by intervaled y. If you replace y: (d) => ((d.height * 10) | 0) / 10 below by y: "height", the box plot shows too many outliers (the outliers are using a map transform).

Here's a test with the correct outliers:

export async function boxplotXInterval() {
  const olympians = await d3.csv<any>("data/athletes.csv", d3.autoType);
  return Plot.plot({
    y: {interval: 0.1, reverse: true},
    marks: [
      Plot.boxX(
        olympians.filter((d) => d.height),
        {x: "weight", y: "height", z: (d) => Math.floor(d.height * 10)}
      )
    ]
  });
}

now, remove the z specified above, and you get this incorrect chart, where you have "outliers" inside the q1-q3 band:

(excerpted from #1839; note that I've updated the code slightly to add y: {reverse: true}, for when we create the actual unit test.)

Start by tracing the map transform and maybeApplyInterval behavior used by Plot.boxX, then compare it with the boxplotXInterval example in the issue. Add or update the unit test using the athletes data and verify that intervaled y grouping produces the correct outliers without explicitly supplying z.