Enhancement proposal

Currently, it can be pretty challenging to a get a broad sense of everything related to graphing in the docs since quite a few methods are under CoordinateSystem, while the rest are under Axes. Also, for example, looking for coords_to_point under CoordinateSystem would suggest that it isn't documented and is NotImplemented, but the real docs are in Axes

There are also some PolarPlane methods that don't really belong under CoordinateSystem.

Here's my suggestion:

• Move methods out of CoordinateSystem and into Axes/PolarPlane (where applicable)
• Maybe make CoordinateSystem private?
• Maybe remove CoordinateSystem all together?
  • Every graph class subclasses Axes anyway.