I created a simple ball bounce: 2 keyframes, ball starts on the ground, springs upward then comes back down to the ground. The 2 keyframes are set when it is at the ground and I used the graph editor to create a curve to get it to be airbound.
First image is the value graph. Makes 100% sense.
Second image is the speed graph. To me the speed graph should look like the value graph (an arc shape) since it goes from acceleration to slow down (when reached the peak height) then it goes back to accelerating.