Wrong position of xlabel in Cartesian subplot mixed with Geographic subplots

[zxdawn]

Description

The location of xlabel is usually in the middle of subplots. But, it doesn't work for Cartesian and Geographic mixed plots.

Steps to reproduce

import proplot as pplt
fig, axs = pplt.subplots(nrows=2, ncols=2, proj=['npstere', 'npstere', None, 'npstere'])

axs[2].format(xlabel='xlabel for Cartesian subplot')

Expected behavior:

The xlabel in the middle of subplot3 xaxis.

Actual behavior:

Proplot version

3.5.1
0.9.5.post202

[zxdawn]

Sorry, I forgot to pass share=0.

[lukelbd]

Think spanning labels across axes with different projections should be disabled automatically. I'll look into fixing that