Fix plot column against itself
My first ever open source PR, be gentle.

- [x] closes #22088
- [x] plotting tests passed 
(310 passed, 4 skipped, 2 xfailed, 4 xpassed, 39 warnings)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
I ran this command and it returned nothing.

This allows for plotting of where `x` and `y` are the same column. I had to add the type check because otherwise it failed a check where `y` is a numpy array.
