[R] plot gam "main effect functions" in one graph

[Pancrazio Bertaccini]

Dear R users,
I have a question about the plot with the package gam.
I need to plot different main effect functions, related to different
gam models, in the same graphics (i.e. the same covariate about
different models).
I used the plot.gam e preplot.gam documentations. Using preplot.gam I
can plot the single function but I'm not able to put all the functions
together.
Does anybody can help me.
Thank you in advance.

Pancrazio Bertaccini,
PhD student,
Università di Torino - Department of Statistics and Applied Mathematics