API & BUG: allow list-like y argument to df.plot & fix integer arg to x,y
- [x] closes #19699
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

@TomAugspurger 

I added support for the user to pass a list-like to y, as discussed in #19699. The API to `df.plot` is relatively complex with lots of args, so lmk with questions / fdbck. Not sure if I covered everything for docs.
