If you feel comfortable with scripting then you may be interested in a quicker way to change the function that is being graphed.
If you are using the "Graph Paper" Card then you can very easily change the "current" function. Hold down the COMMAND and OPTION keys together and press "B". That is the short cut way to edit the script of a background.
Once you have the script editor up you will see something like:
function f x
return [expression]
end f
The expression after the "return" on line 2 of the script is the