BUG: allow numpy.array as c values to scatterplot

Ensure that we can pass an np.array as 'c' straight through to
matplotlib, this functionality was accidentally removed previously.

Add tests.

Closes #8852
@TomAugspurger Is that the expected color? I think the [1, 0] are interpreted as numeric values that are scaled to the default colormap, not as rgb colors? (when you do `c=[[1,1,1],[0,0,0]]` matplotlib does something different)