ENH: Allow scatter plot to plot objects and datetime type data
closes #18755 
closes #30391 

xref #8113

- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Because this is a plot PR, so I put the figures gotten by the two examples in those two PRs in this description to facilitate seeing output, and more cases are tested in test files.
![Screen Shot 2019-12-23 at 7 34 21 PM](https://user-images.githubusercontent.com/9269816/71374603-91982200-25bb-11ea-8ffd-4328fb1ea882.png)



thanks @charlesdong1991 