Add a "gap" option in the stack transform?

Inspiration from https://twitter.com/GRSturge/status/1499000981870288914 : the ribbons are separated by gaps of constant height.

I can reproduce without touching Plot's source code with this plugin:

function gapStackY({ y, gap = 0, ...options }) {
  let y1, y2;
  const ygap = (d, i, e) => {
    const v = y(d, i, e);
    return v > 0 ? v + 3 * gap : 0;
  };
  ({ y1, y2, ...options } = Plot.stackY({ y: ygap, ...options }));
  const { transform: t1 } = y1;
  const { transform: t2 } = y2;
  y1.transform = () => {
    const y1 = t1();
    const y2 = t2();
    return y1.map((d, i) => d + (y2[i] > y1[i]) * gap);
  };
  y2.transform = () => {
    const y1 = t1();
    const y2 = t2();
    return y2.map((d, i) => d - (y2[i] > y1[i]) * gap);
  };
  return { y1, y2, ...options };
}

It's working fine for this use case, even if some options are not fully working (like, y MUST be a function), and I haven't tested with negative values of y. I will research these if you agree it makes sense as an official option.

Start by reading the Plot.stackY transform and use the provided gapStackY plugin as the behavior reference. Determine how an official gap option should handle the shown positive-value case and the untested negative-y case, then verify that the option works beyond the plugin's stated limitations.