prototype channel alias

[mbostock]

This is a quick sketch at allowing channels to be shared across marks as mentioned in #708 (comment). We should think through the syntax and the semantics a bit more.

[Fil]

I made this helper to allow inlining the functionality:

combine = (data, ...rest) => {
  const known = new Map();
  return Array.from(rest, ({ mark, ...options }) => {
    for (let k in options) {
      if (known.has(k)) options[k] = { alias: known.get(k) };
    }
    const m = mark(data, options);
    for (let k in options) {
      if (!known.has(k)) known.set(k, m);
    }
    return m;
  });
}

and the mark becomes:

marks: [
 combine(
  data,
  { mark: Plot.dot, x: "bill_length", y: "body_mass", symbol: "species" },
  { mark: Plot.text, x: "bill_length", y: "body_mass", text: "sex" }
 )
]

[mbostock]

Superseded by #798.