I don’t know if you have already or if you are still maintaining either of these, but if you are, a reasonably simple mod would be combining Parabola with Animated Box for double the awesomeness.
http://squarism.com/2007/04/01/animated-box/

Another cool thing which would tick one of your goals, is adding lua support.
http://squarism.com/questions/

I haven’t done much lua, but you could maybe have a function in your lua file (Pseudo code):
void GetYValue(in x, out& y)
{
y = x;
}

and you can easily add a million variations:
void GetYValue(in x, out& y)
{
y = sin(x);
}

etc.

It’s is kind of limited like that, ie. you can’t get a vertical line. You could extend it to actually read a string so that you can specify either y = … or x = … and your program would magically work it all out.
I’m not sure if lua does it or not, but you could even have a check in your update function that compares the last modified time of the currently loaded script with the modified time of the script on the hard drive and reloads it if it is newer, so you edit the script in XCode or whatever and save it and the graph magically shows up.