Add "end" parameter to coord_polar to allow non-complete polar plots #4462
Description
It would be useful to allow non-complete polar plots. E.g. half or quarter plots. This could be parameterized with coord_polar(start = -pi/2, end = pi/2, direction = -1) and coord_polar(start = -pi/2, end = 0, direction -1) respectively.
This would also allow a more intuitive placement of the radial axis.
Examples:
Half:
Quarter:
