subsetting summary + abline question

["Michael Slattery" <Michael.Slattery@epa.state.oh.us>]

Hello all, 
Again, the depth and speed of this group is amazing! Thanks to L. Gibiansky, 
M. Kurbat. J. Adams, J Gerald, S. Smith, B. Gunther, F. Gosselin, D. Lansky, 
and C. Thacker for their speedy and helpful responses, which converged on using 
tapply, as in:

tapply(SDF18$rslt, list(SDF18$id, SDF18$type), mean)

I have a further question: 
when I create a gui scatterplot, how can I automate a 1:1 line (as I can with 
abline using plot() from the command line). Shy of creating two columns with 
dummy numbers to plot, is this possible?

Thanks again... as a "lesser", I truly appreciate the resource we have here!!

mike