boxplot should materialize its channels?

if we added, say:

  x = valueof(data, x);
  y = valueof(data, y);

then a boxplot of random numbers would be correct. Currently this shows inconsistencies between the different submarks:

export async function boxplotRandom() {
  return Plot.boxX({length: 20}, {x: d3.randomLcg(42), tip: true}).plot({
    style: "overflow: visible;"
  });
}

However it's possible that we would be better off using creator from the pointer-highlight branch to create the box plot as a single mark instead?

Start with Plot.boxX and reproduce the boxplotRandom example using d3.randomLcg(42). Inspect how the x and y channels are consumed by the different submarks, and review the linked pointer-highlight branch for the creator approach. Done means the random-data boxplot renders consistently across its submarks.