What is the constant of proportionality between y and x the graph?

In a right triangle, if you call y and x the two angles that are not 90 degress, then

x + y = 90 ⇒y = 90 - x

Now have present this identity: sin x = cos (90 - x)

Then in our right trianglel sin (x) = cos (y).

This is independent of the measures of the legs.