I don't remember this forum post from three years ago, but I would not say the answer is no. I would say that this would not be particularly easy to implement, and generally would need to be somewhat restrictive. The graph tool does not support general functions to begin with. So the pieces of the function would be restricted to those. Furthermore, it would be quite the challenge just to figure out how the user interface should work for this, as well as implementing that interface.
I also use a multiple choice type problem for questions like this, and at least for my purposes, that is sufficient.