[Solution] : follow mouse movements on "rails"

I wanted to have the possibility to click on something, and while the button isn't released, that the object follow the mouse, but only allong one of the two axis (i.e. when you go to the left or right, the object only slides allong the X axis, and vice-versa).

I needed that kind of behavior for a game, and since there is no pre-made behavior for that, here's a solution : capx