Add support for `autoshift` and `shift` to x axes

[hannahker]

Following from #6334 it should be relatively straightforward to extend the autoshift and shift parameters to support x axes (in addition to current implementation for y axes).

For the most part, the changes in #6334 should work for x axes as well. Adjustments would include:

• Removing and generalizing direct references to y (eg. if letter==='y')
• Reverting this commit
• Mirroring this change for X axis lines
• Extending mocks and tests