Okay. So my understanding is that c, d, O, T, I and G are all constants, the way you got your delta Y.
Since i is vertical and Y is horizontal, then the slope of the graph is
slope = (delta i) / (delta Y)
slope = 1 / [(-f) /(1-c-d)]
slope = -(1-c-d) / f
---if f is big, then the slope is small, and the graph is flatter.
---if f is small, then the slope is big, and the graph is steeper.
---the negative sign only says that the graph is decreasing.
Could it be the arctan(slope)?
Since slope is tan(theta), then theta = arctan(slope) = arctan[-(1-c-d)/f].