NaN handling problems in plots?

[e-koch]

Moved from radio-astro-tools/spectral-cube#215

@keflavich - Matplotlib is handling the nans fine (as in skipping them) so long as zMax, zMin, xMax... are set.

Currently I think the only NaNs come up as radii, so we may want to still plot them using a special marker. This could be done in general for points which can still be plotted in the plane for 2 given parameters, but the z parameter isn't available.