There are two things you will need to do to the Y value 1) Add an offset 2) Multiply by a scale factor.

To determine the values to use you need to know the minimum and maximum Y coordinates of the figure, call these Ymin and Ymax.
As you want the transparency to go from 0 to 1, the values for the offset and scale factor are then as follows :-
Scale factor = (Ymax - Ymin)
Offset = -Ymin

Don't forget to set the Edge and Falloff to 0