New syntax for declaring facets from a mark

[mbostock]

Broken out of #1041 and 0c2fadf.

Instead of this:

Plot.plot({
  facets: {
    data: penuins,
    x: "sex",
    y: "island"
  },
  marks: [
    Plot.dot(penguins, {
      x: "culmen_length_mm",
      y: "culmen_depth_mm"
    })
  ]
})

You could say:

Plot.plot({
  marks: [
    Plot.dot(penguins, {
      x: "culmen_length_mm",
      y: "culmen_depth_mm",
      fx: "sex",
      fy: "island"
    })
  ]
})

Or in shorthand, instead of this:

Plot.dot(penguins, {
  x: "culmen_length_mm",
  y: "culmen_depth_mm"
}).plot({
  facets: {
    data: penuins,
    x: "sex",
    y: "island"
  }
})

You could say:

Plot.dot(penguins, {x: "culmen_length_mm", y: "culmen_depth_mm", fx: "sex", fy: "island"}).plot()

[mbostock]

If multiple marks specify _fx_and fy, what happens? It means that marks can use different data, but share the same fx and fy scales. In other words, the domains for the fx and fy scales will need to be unioned across all marks that define fx and fy channels (just like the other scales). So this:

Plot.plot({
  facet: {
    data: penguins,
    x: "sex",
    y: "island"
  },
  marks: [
    Plot.dot(penguins, {
      x: "culmen_length_mm",
      y: "culmen_depth_mm"
    }),
    Plot.text(penguins, {
      x: "culmen_length_mm",
      y: "culmen_depth_mm"
    })
  ]
})

Is equivalent to this:

Plot.plot({
  marks: [
    Plot.dot(penguins, {
      x: "culmen_length_mm",
      y: "culmen_depth_mm",
      fx: "sex",
      fy: "island"
    }),
    Plot.text(penguins, {
      x: "culmen_length_mm",
      y: "culmen_depth_mm",
      fx: "sex",
      fy: "island"
    })
  ]
})

And likewise, this:

Plot.plot({
  facet: {
    data: penguins,
    x: "sex",
    y: "island"
  },
  marks: [
    Plot.dot(penguins, {
      x: "culmen_length_mm",
      y: "culmen_depth_mm"
    }),
    Plot.text(penguins, {
      x: "culmen_length_mm",
      y: "culmen_depth_mm",
      facet: false
    })
  ]
})

Is equivalent to this:

Plot.plot({
  marks: [
    Plot.dot(penguins, {
      x: "culmen_length_mm",
      y: "culmen_depth_mm",
      fx: "sex",
      fy: "island"
    }),
    Plot.text(penguins, {
      x: "culmen_length_mm",
      y: "culmen_depth_mm"
    })
  ]
})

And then the cool thing, if I want to have e.g. one label per facet, I no longer have to use the selectFirst transform; I can instead provide data whose fx and fy channels map to the expected values.