3D Scatterplot: Add multi-variate cluster option

```
an idea for the 3D scatterplot after seeing Ross' program where he uses
shortest-distance cluster algorithms (like k-means and others) to identify
similarity of multivariate coordinates. We could add something like this as a
checkbox option to the 3 variables specified in our 3D scatterplot and then
color-code any resulting cluster in the 3D scatter space and the map. Could
also add a third X to the weights since we already allow users to specify
non-geographic XYs in the weights dialog.
```

Original issue reported on code.google.com by `jkosc...@asu.edu` on 6 Mar 2013 at 1:15

Start by locating the 3D scatterplot and weights dialog entry points, then review how the three plotted variables and non-geographic XY weights are currently handled. Done would mean defining the clustering option, supporting multivariate clustering, and color-coding the resulting clusters in both the 3D scatterplot and map.

Written by the indexing model from the issue text.