BUG: Fix timezone-related indexing and plotting bugs
- [x] closes #15552
- [x] closes #20441
- [x] closes #17173
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


1 timezone plotting bug, 1 indexing bug with tz data, 1 timezone data regression test
thanks @mroeschke 