df.plot() with kind='scatter' and datetime on x axis bug
I noticed while helping a student that there's a bug when calling a scatter plot as a method on a DataFrame. Regardless of what the index is or what the x argument is, it appears that pandas makes the x argument into the index, and then searches for the x in the columns, where it no longer exists. When calling the same directly through matplotlib as plt.scatter, it worked properly.

<img width="1074" alt="Pandas_Error_scatter" src="https://user-images.githubusercontent.com/35703276/71311989-e7ba6900-23f3-11ea-9911-ebf4e2b1c463.png">
