Histogram as color key

The examples on the [holoviews.org](http://holoviews.org) homepage show how to use a histogram as a color key, which would be really useful for datashader since we are working with very non-uniformly distributed datasets that are much easier to debug if we can see the mapping between the data distribution and the color distribution in the image. It would be great if we could have not just a color key of magnitudes, as in the new dashboard.py, but also a histogram like this. Note that it needs to be a special type of histogram for this to work, with a little line indicating zero and showing some of the bins below zero, or else low-count bins won't have any visible color (again, see holoviews.org for the example).