support kernel density plots in 2d and 3d

[jeremymanning]

write a function for creating kernel density plots, along the lines of seaborn's kdeplot: http://seaborn.pydata.org/generated/seaborn.kdeplot.html

in 3d, each group's distribution would be plotted as a series of concentric 3d shapes

to figure out: what would be a good syntax for using this mode?

[jeremymanning]

example plot:

(from http://seaborn.pydata.org/_images/seaborn-kdeplot-9.png)

[andrewheusser]

i think these may look weird considering the matplotlib 3D plotting issues. wondering whether its worth considering switching to mayavi under the hood / how much work that would actually take:

http://stackoverflow.com/questions/25286811/how-to-plot-a-3d-density-map-in-python-with-matplotlib

[jeremymanning]

It may not be too hard as long as we're willing to stick with the simplified styling and not have axis labels...

[jeremymanning]

We could also implement things directly with OpenGL, e.g. using pyopengl

[jeremymanning]

Or we could switch to plotly...