VIS: Fix DataFrame.plot() produces incorrect legend markers when plotting multiple series on the same axis
I found out that the reason this happens is because `subplots` is not updated accordingly when `plt.subplots()` is used and assign to `ax`, e.g. `fig, ax = plt.subplots(nrows=1, ncols=3)`. I tested my solution and things all get solved, and figure looks very correct like below:
![Screen Shot 2019-08-07 at 10 03 39 PM](https://user-images.githubusercontent.com/9269816/62654665-95a30a80-b960-11e9-94ad-ab43a0e8a3cf.png)

BTW, i am not very sure how to properly test this, and would be nice to know!

- [x] closes #18222
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
