For each pair of curves, you will find blue(i)+black(i)=constant.

So it is corresponds to conservation of energy.

You can also draw curves for (1/2)m*v[sub]i[/sub][sup]2[/sup] and m*g*y[sub]i[/sub], and you will find the sum are the same for all the curves.

What I did was just change the unit to be (m*g). So I draw (1/2)m*v[sub]i[/sub][sup]2[/sup]/(m*g) and y[sub]i[/sub]-- vertical position of particle.