BUG: Enable plotting with PeriodIndex with arbitrary frequencies, resolves #14763
- [x] closes #14763 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Enables plotting of data with PeriodIndex with frequencies which are multiples of the `data.index.freq.rule_code`.
Hopefully I'll find some time to include tests within the next 3 weeks. Since this is my first pull requests, I may need some additional time to write tests.