Thanks a lot for your feeback! We are really happy that you enjoyed our game.
We uploaded a new version yesterday with some minor bug fixes, this version also includes the ball trajectory feature and an extended repertoire of questions. We got to the same conclusion at the beginning as you did when implementing the arrow over the trajectory, "it may be too easy with a such a visual assist", we'll keep an eye on the new feeback to decide which one fits better. Having the possibility to enable or disable it may be a good idea too, we'll take note of that.