ENH: scatter plot support for colormap

[pratapvardhan]

See: pandas.DataFrame.plot.scatter

df = pd.DataFrame(np.random.rand(50, 4), columns=['a', 'b', 'c', 'd'])
df.plot.scatter(x='a', y='b', c='c', colormap='viridis')

If this looks good, I'll update tests.

[jakevdp]

Looks good. One concern I have is that the names of matplotlib colormaps do not correspond to the names of vega schemes, but there's not really any easy way around that.

[Casyfill]

can we for all those params support both strings and alt objects?

[jakevdp]

can we for all those params support both strings and alt objects?

I'd lean toward no. I think the main benefit of this API is the ability to create charts that basically work on multiple backends. If we start supporting altair-specific extensions of the core API, then that flexibility goes away, and it may not be apparent to users when they have created a chart that will no longer render with one of the other backends.

[pratapvardhan]

@jakevdp how would you like to proceed on this?

[jakevdp]

Let's go ahead and apply colormap directly to scheme as here. Not a perfect solution, but I think it's the best thing that can be done easily at this point.

First tests and a couple minor changes.