d3.geoRotation about d3-geo

[mbostock]

We’ll break the circular dependency. d3.geoRotation only depends on equirectangular out of convenience to define the inverse rotation. Instead of this:

d3_geo_identityRotation.invert = d3_geo_equirectangular;

It should be sufficient to say this:

d3_geo_identityRotation.invert = d3_geo_identityRotation;

[chrisuehlinger]

Just a heads up, we're going to run into a circular dependency between d3.geoRotation, equirectangular and d3.geoProjection. If we're planning on breaking d3.geoProjection into a separate node module, that would also mean that people importing these modules could end up with a circular dependency (which could be problematic depending on how they bundle/load).

It's looking like the most straightforward way to resolve all of this is to keep d3.geoProjection in d3-geo (along with the "batteries included" projections like spherical and equirectangular, which are required in components like d3.geoPath and d3.geoCircle) and reserve d3-geo-projection for the more exotic projections (basically, the way things are now).

If you feel strongly that we should separate d3.geoProjection into d3-geo-projection, I'm cool with making that the focus of my next large effort. However, I do wonder whether there's much of a use case for d3-geo without any projections (you can't really render a map without them).

[mbostock]

Fixed in #18 by @chrisuehlinger.