You have to compute the distance between two position.
distance = sqrt((x0 - y0)2 + (x1 - y1)2)
(if you want an external look at my tools or the zsa objects, there are objects that compute distances)
then compute the velocity :
velocity = distance/time
The most difficult is to know wich values to take. I think the best is to take the values when y is highest for time 0 and lowest for time 1.