Help request for trellis panel function

[David Parkhurst <parkhurs@imap.iu.edu>]

I have a data frame with five columns, x, y, A, B, and C.  x and y are 
continuous numeric variables, and the others are factors.
A has 5 levels, B has 2, and C has 6.  I'd like to produce a 5X2 trellis 
of xyplots, using different symbols for the different values of factor 
C.  In addition, I'd like each of the ten panels to include a 1:1 line.
I know this would require a special panel function, but I don't know 
what it should look like.  I would much appreciate help in creating it.
Thank you.

David Parkhurst