Add distribution sugar to scatterplot

[yelper]

When looking at multi-class scatterplots, it can be helpful to add components such as box-plots or distribution curves for each class, aligned with the axes of the plot.

Placeholder bug to gather ideas, particularly for how they should look or should act.

[yelper]

Could do something like the histogram and KDE plots at the bottom of this link: http://seaborn.pydata.org/tutorial/distributions.html