After effects speed data in graph editor not coherent

Question

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:

Answer 1

You're making animating a bounce much too difficult. Way too much room for funky behavior.
Set your layer at it's starting point, set a position keyframe, then move the layer to it's ending point but keep it at exactly the same height (y value). Now add this expression and adjust the Frequency, amplitude, and decay to get the path you want. The amplitude is the number of pixels you want the ball to fall.
freq = 1.0; //oscillations per second
amplitude =  90;
decay =  .5;
posCos = Math.abs(Math.cos(freq*time*2*Math.PI));
y = amplitude*posCos/Math.exp(decay*time);
position - [0,y]
Your graph will then look like this:
and you'll have a much more realistic bounce.
Thank Dan Ebberts for the expression with a very slight modification by me. BTW, save this as an animation preset and you've always got it when you need it.