BUG: Plotting use FixedLocator
For string x values or multiindex, the axis locator is now set to a
FixedLocator for Line and Area plot. The ticklabel stay consistent
when scaling and zooming.

closes #7612 
closes #15912
closes #22334
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
