Natural-Selection | 2D Platformer made using the Pygame Library | Game Engine library

The essential question of what I am trying to ask is: How would I make a graph in which the line at y=0 is actually at y=*some number* and that as my Y increases, Java's y decreases?

I am attempting to make a line graph which documents change of populations of animals. I need to make the graph's Y=0 around the y=980 line. I also need to make something that would notice an increase in population and graph it as a decrease in y (to make the line go UP). What I'm trying to say is that I need to create a line graph that looks like a line graph.

I have attempted multiple different things, which each give me a different result based on different inputs. I have successfully created the graph that starts around the y=980 line and goes up, as shown by the image below. The method I used for this was to draw the lines (for the graph itself) and then to take the absolute value of the difference of 10 times the value minus 90, as shown by