better error message for missing stack value

[mbostock]

Originally, I tried this:

Plot.rectY(
  series,
  Plot.pointerX({
    x: "time",
    y: null,
    interval: {
      floor: (x) => Math.floor(x) - 0.5,
      offset: (x) => x + 1
    }
  })
),

This leads to a vague error:

TypeError: object null is not iterable (cannot read property Symbol(Symbol.iterator))

The error can be reproduced as:

Plot.valueof([], null, Float64Array)

In the context of valueof, I think the error is okay: if you specify an array type then (I guess?) the value argument must not be null. But in the broader context of the stack transform, the stackY transform requires that y not be null since it will subsequently dereference those values even if we allowed valueof to return null here. So I think it’s better that we abort early with a more helpful error message.

Eventually I realized I could use rect instead of rectY, and by not specifying y, the rect spans the vertical extent of the chart and I don’t need rectY’s implicit stacking. I think this makes a nice test based on #1795.