BUG: Spurious IncompatibleFrequency error prevented plotting of non-standard intervals (Fixes #14763)
 - [x] closes #14763 
 - [x] tests added: tests/plotting/test_series.py: test_non_standard_intervals
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry - Confirm version and I'll add

Fix Summary: A check on frequency compatibility was unnecessarily strict. Now IncompatibleFrequency exceptions will only get raised if the unit types differ.