After effects speed data in graph editor not coherent

Hi, im animating the typical bouncing ball, and i have activated separate dimenstions to animate X,Y separate. So i add two keys on the X value that has a constant speed, then i animate the Y value, but i notice that the speed goes below zero in the graph editor and i dont understand it..
Should it not be Zero as the ball stops? (Green line)
Here you have an image with my situation:

http://imageshack.us/photo/my-images/502/aespeed.jpg/

One thing to keep in mind is that this is NOT a graph of the position values of your ball, rather it is a graph of the velocity values of your ball, or a graph of how the position of the ball is changing. Were you to have XY combined, this would be, in essence, the speedometer for the ball -- if the values are up high, then the ball is moving fast. So at the moment when the ball hits the ground, the graph would be at its HIGHEST point, because the ball is moving fastest just before it hits. Likewise, the graph will be at its lowest when the ball is at the peak of it's bounce, since that is when it is moving the slowest, waiting for the next fall, as it were.

But that's with XY combined, which forces AE to do vector math to produce the graph.

With the XY separated, you lose the speedometer effect. Now there is only a single value to plot, so there can be a positive and negative value, whereas with XY combined there is only moving / not moving (nothing goes below zero when they are combined, in other words). So X holds steady at 400 pixels per second to the right (thus a positive number -- right is positive, left is negative, in AE-land).

When Y is moving down the number goes up. It's not very intuitive (not for me, at any rate) but it helps explain some of the confusion, I think. Your ball experiences a sudden burst of upward thrust when it hits the ground -- that's when the green graph line jumps down. From that point on, the ball is being constantly pulled by gravity, so there is a constant reduction in speed, thus the graph slowly (and steadily) moves up (remember, everything is backwards!).

When your line crosses the 0 point while moving up, that is the moment when your ball changes from traveling up to traveling down. Phew! You wouldn't think that zero line matters too much, and mathematically the graph may not care too much, but visually it makes a big difference when the Y crosses that zero point, even if it does so with such a smooth diagonal line....

Ben Unguren
Motion Graphics & Editing
http://www.mostlydocumentary.com