I have been using Graph for years now and it has really came a long way, although I am an experienced engineer I still love the simplicity of your program (and for doing a quick plot it is the most used program in "my arsenal").
Since Matlab is in one of my beloved programs there are some features that I miss in Graph (and vice versa, of course). One thing, that really bothered me since high school was axis scaling, so here is a suggestion:

Problem occours when plotting very "dense" functions, like 80*sin(50x). Add a key, that will scale the axis in such way that a few x interceptions and the whole height of the function can be visualised (ex. scale y= 0-100, scale x=0-1).

Keep up the good work!

steel_mfg