[R] Adding points to a wireframe with conditioning variable

[Mark Lyman]

I would like to add points to a wireframe but with a conditioning variable. I 
found a solution for this without a conditioning variable here, 
http://finzi.psych.upenn.edu/R/Rhelp02a/archive/65321.html. Does anyone know 
how to plot a wireframe conditioned on a variable and add the points 
conditioned on the same variable, similar to the solution at the link above?