BUG: Fix bad axis limits when boxplotting with 'by' (#7528)

closes #5517
 
The original bug involved the y-limits for boxplots being wrong when using `'by': turns out it has to do with the dummy axis that gets created when the plots don't fill the axis layout, e.g. when we're plotting 3 columns in a 2 by 2 layout.

The fix for this basically involves not setting the `sharey`/`sharex` argument for blank/dummy axes.

I've tried to test this in a couple of different ways, but some feedback on better ways to test would be good. In particular, I'm not sure if it's okay to check the `_sharey` attribute of the axes, given that it's private.