QTR-Quadtone RIP

When you hit the Create Curve button in Curve Creator menu, it brings
up a graph depicting the inks in your curve. 

I'm trying to understand what this graph is showing. It runs from
0-100 both up/down and left/right but these are not labeled. 

I'm guessing it has to do with how much in is being laid down, or at
what point in the tonal scale the inks start or stop printing, but i'm
not sure. 

Can some maybe explain in simple terms. 

thanks, David