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
>c can be a single color format string, or a sequence of color specifications of length N, or a sequence of N numbers to be mapped to colors using the cmap and norm specified via kwargs (see below). **Note that c should not be a single numeric RGB or RGBA sequence because that is indistinguishable from an array of values to be colormapped. c can be a 2-D array in which the rows are RGB or RGBA, however.**

from http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.scatter

We should handle the same if possible.