2.) The domains on these graphs represent the amount of time elapsed during the skateboard's movements. The ranges of the graph represent the distance traveled by the skateboard in each period of time.
3.) As the ramp gets shorter the maximum of the graph gets lower. This is because with a shorter ramp the initial momentum of the object is lower. All three graphs start at zero for obvious reasons.
4.) The graph rises the fastest in the very beginning because the object has a greater velocity and falls the fastest right after the turn around because it have the greatest momentum.