Add cmap argument to pairplot

Tell us about it

The coloring of a hexbin plot is controlled (at least with the matplotlib backend) by a cmap argument. This argument is not exposed to the programmer through the pairplot parameter set, and the backend plotting function ignores the backend_kwargs argument, so the programmer cannot control this except through setting the cmap parameters of the backend, potentially breaking other plots.

Thoughts on implementation

Probably the simplest method is to start using backend_kwargs instead of supporting cmap directly. Supporting cmap directly would be nicer, though, since it doesn’t require understanding the backend, and it would expose the cmap to the user in the docstring.

[canyon289]

@rpgoldman can you confirm this is fixed?

[OriolAbril]

Similarly to the contour case, it should be passed via hexbin_kwargs={"cmap": "plasma"}. Does this work?

backend_kwargs are passed to plt.subplots at figure-axes creation