Driver for UV Map activeness

I am trying to create drivers to animate the active UV Map. I have created a driver for each maps activeness. Its a scripted expression based on the local Z location of a cube. I would like for the end result to be…

The cube goes up 1 Z map 1 is active.
The cube is at 0 Z map 2 is active.
The cube is a -1 Z and map 3 is active.

It seems like I should change the X Y in the graph editor for each driver but thats where i am stuck. I dont know why its not working.

Its my understanding that in the graph editor (X) is the input and (Y) is the out put. I am just assuming 0 is off and 1 is on.

so…
Map 1 _ X1 Y1
Map 2 _ X0 Y1
Map 3 _ X-1 Y1

But how do I get it to go back to 0?

I am wrong but I dont know why…