Request for Additional Documentation: Creating new coords

[elbamos]

There is currently extensive documentation on using the proto system to create new geoms, facets, and scales. However, there isn't for creating new coordinate systems. I'm trying to extend/modify Coord_Polar, and its quite challenging trying to determine what needs to be implemented and what does not. Some documentation would be greatly appreciated.

[paleolimbot]

This might have to be coupled with exporting some coord-related functions (like possibly some of the expansion functions added in #3380). I seem to remember that extending a coord was nearly impossible without copying some internal ggplot code), and I think that documenting the extension process might help decide which of functions need to be exported.

[teunbrand]

I have found it hard to come up with a didactic example that touches important parts of the Coord class without becoming very involved with all sorts of details and loosing sight of the forest for all the trees. It is not too difficult to come up with complicated examples or easy examples, but the sweet spot is hard to find an example for.