determining the order in which points are plotted

I make plenty of scatterplots, especially using scatterplot.matrix  
from library(car).  One thing I don't know how to do is determine  
which points are plotted last.  Sometimes I plot a large number of  
points for multiple groups represented by different colors.

I would like to guarantee that point that are far from the centroid  
for their group are plotted last.  This way they will be visible  
because they won't be buried under a pile of points from another  
group.

As it stands, it looks like scatterplot.matrix (and maybe other  
plotting functions) lay down a group at a time, in order, so that  
the first group is most likely to be buried under later groups.  I  
try to sort factor levels so that the biggest groups go first, and  
that helps a little, but it isn't the complete solution I'm looking  
for.

Thanks in advance.

Mike

--
Michael B. Miller, Ph.D.
Minnesota Center for Twin and Family Research
Department of Psychology
University of Minnesota