facet: "first"?

[Fil]

Sometimes we want a mark to happen in the first facet (say, a guiding annotation).

Currently this can be done by making that mark's data "align" with the original data so that it falls on the same index, typically:

Plot.text(["annotation"], {facet: true})

which is fragile and a bit difficult, depending on the number of data we want to display and the faceted data. It could be nice to say {facet: "first"} instead. Another (separate and additional?) possibility would be to say facet: ["category1", "category2"] and have the mark appear on these two facets (only).

cc: @ericemc3