Plot.rule + projection for meridians and parallels

[mbostock]

If Plot.rule supported projection, like Plot.line does, we could use Plot.ruleX to render meridians and Plot.ruleY to render parallels. That’d be nice.

[mbostock]

A rule typically extends across the opposite domain; e.g., a ruleY spans the domain of the x-scale. So when there is a projection, we would need to know the domain of the projection to know the extent of the rule (e.g., from -180 to +180).

And likewise we need to know whether the projection is spherical, because if it is, then ruleY will not produce geodesics and will need to be sampled with some precision to produce lines of constant latitude.

This suggests that we may need to expose additional information about the projection to marks. But this is not urgent because we already have Plot.graticule and we could easily add more convenience marks for spherical geometry.