Shapes and Morphing - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 292

In the case of Drivers, a modifier called Expanded Polynomial is added by default. We're not going to
go into it in detail, but its effect is to replace any curve in the Graph Editor with a perfectly diagonal line
that provides the original driver 1 : 1 relationship. We want to send that relationship packing, so click the
“X” to kill the modifier. While we're removing stuff, let's completely isolate the “open jaw” curve in the
Graph Editor. On the left panel where the different objects and animation channels are listed, uncheck all
of the boxes beside them, with the exception of the “Value (open jaw)” one. You can uncheck all of the
armature-related ones with a single click, by removing the check from beside the Armature channel itself.
When you're done, only the “Value (open jaw)” channel should be checked. There will be no curve in
the editor's workspace, because you already removed the Modifier that was responsible for generating it
in the first place.
Ctrl-LMB click twice in the Graph Editor's workspace. This will create a two-point curve, possibly like
the one in Figure 10.10 .
Now comes the math. When the bone is at 0.0 along its z axis, we want the shape key to also have a 0
value. Unfortunately, there's no way to directly enter x and y axes values for curve points in the Graph
Figure 10.10 Any two points will do.