Follow path movement seems to have no editable curve

It’s been a while since I used Blender so I may be being an idiot.
I am animating a simple ball rolling down a stair, I didn’t succeed doing it with physics (blender physics never seemed to work for me) so I made a path and told the ball to follow it.
However to make it look like it’s realistically(ish) falling, the speed would need to vary, getting faster as it gets to the actual falling part. I thought I could just go into graph editor and edit the f-curve, but there isn’t one…or rather there is one red line but it isn’t editable in any way.
Anyone can help?

blend file:
staurfdewball.blend (804 KB)staurfdewball.blend (804 KB)

It would probably easier to animate the movement of the ball using a follow path constraint on the ball instead of animating evaluation time. You will need to remove the evaluation time f-curve before adding keyframes to the Offset of the follow path constraint. Here is an example. I also added rigid body physics to another ball.

staurfdewball1.blend (921 KB)