BUG: Plotting Timedelta on y-axis #16953
- [x] closes #16953 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This fixes the issue with a TypeError being raised if the y-data were of type Timedelta or Datetime.
The plot of Timedelta doesn't look pretty since it is converted to ns representation, but matplotlib has already an open issue for that.

PS: Greetings to the people from the EuroSciPy 2017 sprints ^^
