Qstatistic.plot does not return axes nor take axes

[ljwolf]

It's challenging to use QStatistic.plot alongside contextily, since the method

1. does not allow you to pass an ax
2. it constructs an axis (in self.pp.plot()), rather than requesting an axis using plt.gca()
3. it calls ffigure.show() rather than returning figure/axis components that can be modified after plotting.

This means we can't add basemaps, as far as I can tell.

[ljwolf]

I think, looking at many of the plotting methods in pointpats, this is a possibility for enhancement. It should be pretty straightforward if we use matplotlib-style returning of figures/axes & taking an ax argument.

[martinfleis]

Closed by #116