follow-up: derive x & y scale domains from geometry

[Fil]

addresses TODO items in #1468 and adds/tweaks tests

[mbostock]

I think we’ll need a fancier test than projection == null, per this comment on projectionAspectRatio:

plot/src/projection.js

Lines 225 to 241 in df3a3ad

	// When a named projection is specified, we can use its natural aspect ratio to
	// determine a good value for the projection’s height based on the desired
	// width. When we don’t have a way to know, the golden ratio is our best guess.
	// Due to a circular dependency (we need to know the height before we can
	// construct the projection), we have to test the raw projection option rather
	// than the materialized projection; therefore we must be extremely careful that
	// the logic of this function exactly matches Projection above!
	export function projectionAspectRatio(projection, marks) {
	if (typeof projection?.stream === "function") return defaultAspectRatio;
	if (isObject(projection)) projection = projection.type;
	if (projection == null) return hasGeometry(marks) ? defaultAspectRatio : undefined;
	if (typeof projection !== "function") {
	const {aspectRatio} = namedProjection(projection);
	if (aspectRatio) return aspectRatio;
	}
	return defaultAspectRatio;
	}

A quick take, but should have more thought:

function hasProjection({projection} = {}) {
  if (projection == null) return false;
  if (typeof projection.stream === "function") return true;
  if (isObject(projection)) projection = projection.type;
  return projection != null;
}

But the bad part is that the projection can be specified as a function (a “projection initializer”) which can return null indicating that there isn’t actually a projection. But maybe we don’t care about that nuance here, since this is really just a performance optimization and shouldn’t change the behavior?

[Fil]

I don't think we even need to cover the projection: null and projection: {type: null} cases, tbh. Having a smart domain when projection===undefined would be enough to solve the casual use of Plot.geo with some geojson. If the user starts to specify a projection, then they know what they're doing.

[Fil]

Hmm it was the other way round: we can only skip that geometry look-up if the projection is guaranteed to be non-nullish. Which is what you were saying :)