I did not study any Business Math. I use only "common sense" or reasoning in trying to solve "business" problems.

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

Therefore,

---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.

"couple"?

Beats me.

angle?

Could it be the arctan(slope)?

Since slope is tan(theta), then theta = arctan(slope) = arctan[-(1-c-d)/f].