Change DELTA (change in x) to a graphical equation

hi. I am a biologist - and therefore lack basic math ability. I have an equation that predicts a curve. Its Delta y = (Max y - y)* a. I want to use sigma plot to do draw this line - I'm changing the value of "a" in my experiment - so i have a set of observed results and i want to compare them to the predicted from this experiment. How do i change the above equation into one that i can input into sigmaplot/sas/spss to draw the line. I want something like

y = ........

So basically i think i want to go from "Delta y" to just "y"

cheers