[Feature request] span axis labels

[cvanelteren]

Description

I often have the issue where the axes are not shared, but I do want to share the labels. For example when I am mixing some axes and not others. I propose to add a feature where you can specify the sharing of the axis. As far as I am aware this is not available in proplot. Currently I resort to something like

def add_main(fig, nrows, ncols, span):
 main = fig.add_subplot(nrows, ncols, span)
 main.set_facecolor("none")
 main.set_xticks([])
 main.set_yticks([])
 for spine in main.axes.spines.values():
     spine.set_visible(False)
 return main

fig, ax = plt.subplots(ncols = 2, nrows = 2, share = 0)
main_top = add_main(fig, 2, 2, (1, 2))
main_top.set_xlabel("group 1", labelpad = 25)
main_top.set_ylabel("group 1", labelpad = 25)

main_bottom = add_main(fig, 2, 2, (3, 4))
main_bottom.set_xlabel("group 2", labelpad = 25)
main_bottom.set_ylabel("group 2", labelpad = 25)

fig.show()

Resulting in

Some of it can be achieved with fig labels. If this can already be achieved, we could add it to the docs, if not we can add it.