Thanks. The answer is pretty obvious once you know what it is. As far as I can tell, this is not DIRECTLY addressed in the manual. But, since the manual (.pdf file) is 887 pages long, it is very likely that I just missed it.

I must stand corrected, the trick still works on the 48S and 48G and even on the 49g+/50g+. You have to store something like 'SIN(X)=COS(X)' in EQ and can than plot both curves simultaneously.