BUG: fixes #12405 by eliding values index by NaT in MPLPlot._get_xticks
- [ ] closes #12405
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Proposed solution: elide rows where x value would be `NaT` in [MPLPlot._get_xticks](https://github.com/pandas-dev/pandas/blob/7f5a45c1b388b3f7f309f82bfa0733b7b9980c3a/pandas/tools/plotting.py#L1328) (which seems like a minor misnomer: its getting x values, not what will ultimately be ticks).

Reordering occurs in MPLPlot._get_xticks and it has a block that is conditional on `use_index` and `is_datetype`, so seems like a reasonable place to elide the values.  Mutation of `self.data` already occurs there (reordering) and a read through suggests nothing has saved info related to `self.data`'s shape before this point.
