BUG: Fixes color selection in andrews_curve

Fixed bug introduced in b5265a5b84ae1aeaf5fff88f1f9e5872a9b404e6 which change from picking a single random color for the class to picking a random color for each line.

This returns the color selection to a per-set basis.  

See http://stackoverflow.com/questions/19667209/colors-in-andrews-curves