Horizontal axis to touch input

Hello,

The player in my game moves on the horizontal axis only. So far it's moving via left/right arrows that pass Input.GetAxis("Horizontal") to my MovePlayer() function. It's a mobile game so I'd need to obtain the same effect with touch input.

Ideally it should work like an invisible analog stick that detect horizontal-only touch across the whole screen. Can you help me figure out how to make this happen? thanks....