Plotting: don't change visibility of xaxis labels and ticklabels if passing in a axis
This does two things:
- fix for changing the wrong xlabels/xticklabels if a gridspec is used and the ax is passed in (not sure about using subplot without gridspec)
- changes the default behaviour for sharex if an axis is passed in: before the visibility changes were applied per default, now they are only applied if explicitly requested via `sharex=True`

Closes: #9737
