Wolfram|Alpha: Computational Knowledge Engine
have you tried this one, not sure tho if this meets your request.
Hi, whilst not strictly mathematics, is there any program, website etc. in which instead of plotting points and a line graph is drawn from those, I can draw a line graph physically (with a pre-determined set of axes) and a set of points are generated? I'm not even sure such a thing exists but was just very curious, I would almost describe it as the reverse of a conventional graph. Thanks!