Make Plot more geojson-aware

[Fil]

Only the geo mark knows that data can be a FeatureCollection of a GeometryCollection. This makes pivoting from Plot.geo to Plot.dot a 3-changes operation:

Plot.geo(countries) => Plot.dot(countries .features, Plot.centroid())

In the spirit of #1743/#1744 we could have all the marks:

1. accept geo collections as data, and add a geometry: d => d option
2. apply the centroid initializer if they consume x, y and there is a geometry (and no x, y or frameAnchor); similarly, apply a bounds initializer if they consume x1, y1, x2, y2 and …

[mbostock]

I don’t understand the rationale here and this feels like it will be a bit too magical. Why not use the geo mark, or why not let the user handle the mapping from geo to point/bounds?

[Fil]

I see this happen a lot during pairing sessions and workshops: people (me included) often start from the geo mark on a geojson, then want to duplicate the mark and change it to something else (e.g. a dot or a text label).

There's often a moment of doubt when nothing shows up on the screen. You have to remember to:

• get a point (centroid, geocentroid, pole of inaccessibility…)
• get the features as data, instead of the collection that worked in the geo mark
• also, find the values in the properties (d => d.properties.name instead of "name") — this is covered by Shorthand for GeoJSON properties #1172

I feel that it would be helpful if (some of) these were automagical, in cases where there is no ambiguity. For instance, a {type: "FeatureCollection"} object can't be a data iterator.

I understand that the application of the centroid to get x,y is a bit more difficult (since we can have points defined without x or y, e.g. when we're using frameAnchor), but it doesn't seem impossible.

[mkfreeman]

I've run into this a number of times, finding it easy to forget to append .features to a mark's data when using a centroid transform.

I would have expected centroid transforms -- perhaps only if there is a projection? -- to be geoJSON aware, using logic like this line from the Plot.geo mark:

  switch (data?.type) {
    case "FeatureCollection":
      data = data.features;
      break;

That being said, I'm not familiar with how (if) transforms make checks like that.

If nothing else, it would be nice to add a warning when Plot.centroid is passed a geoJSON object.