BUG: fix custom xticks in bar() plotting function
- [x] closes #14119
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

@GiuPassarelli can you merge master and fix conflict? I think just one outstanding comment here otherwise right? Thanks @GiuPassarelli!