API: Improper x/y arg given to df.plot
- [x] closes #18671 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry 

I'm not incredibly familiar with your codebase so I did my best to follow conventions. Not sure what to do with whatsnew for this case?

Description:

- Validation of x or y arg to `df.plot` to match specifications in documentation
