How do I copy a certain axis for a certain number of frames?

I want to copy only the Y axis for 15 frames, its a ball bouncing, I already set up it going through the x axis, so right now its basicly a ball going from 5 units up on the Y axis, going down then going in a straight line(on the x axis), I want to only copy the amount it goes down on the Y axis not how much it moves on the x axis. I can of course do it manually but I'd just like to know for future refrence.

I really hope you can understand that, I'm not very good at explaining, sorry.

Thanks alot, I appreciate it.