REGR: Bar plot axis timestamp to string conversion error
Added regression test to prevent bar plot from attempting to convert x-axis from string to timestamp due to previous plot's axis units.

Problematic example:
https://github.com/pandas-dev/pandas/blob/fb353446651067fafec03301256e93d59065c520/pandas/plotting/_matplotlib/core.py#L1433


- [x] closes #38736
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @ivan-ngchakming 