Here is a way to construct Interaction Plots. It gives you a plot of the average values of y for each period and trt . Sometimes it can be very handy in shortening output. You can do plots by another variable. Here is a fancier way to construct Interaction Plots and some Diagnostic Plots, which allows you to use the ful value of statistical modelling.
Diagnostic information is saved with the output phrase.
The third example following takes data from file hatps.dat , creates a 3-D plot, and outputs it to a postscript printer. I am not sure if this is saved as a file, or sent directly to one's default print device.