MEP24: negative radius in polar plots




It is clear that polar plots need to be able to gracefully handle negative r values (not by clipping or reflection).

Detailed description

One obvious application that we should support is bB plots (see, but this seems more generally useful (for example growth rate as a function of angle). The assumption in the current code (as I understand it) is that the center of the graph is r==0, however it would be good to be able to set the center to be at any r (with any value less than the offset clipped).