[R] overlaying several subsets of data frame in pairs plot

[Marion Dumas]

Hello
I have a multivariate data frame giving various responses for several  
treatments. I would like to plot the relationship in the responses in  
a pairs plot with different symbols for the different treatments. In a  
regular plot I would have used  'matplot' or just added the new  
treatments with the 'points' function. But how to do it in a pairs plot?
Thank you for your help
Best
Marion Dumas