[HELP] Trying to set up a gravity (or magnetic) plane running through the seen instead of downwards

Bat and ball game

I want the ball to want to head towards a middle plane on the XY. If I hit the ball up and forward [Z,Y], the gravity plane means the ball will traverse an ever decreasing sine-wave (up and down) along the y axis. The left or right should not be affected. See image.

Idealy this plane will be invisible with zero collision.

Is this possible?