How to determine the equivalents of OH- in a titration curve

How does one go about determining the equivalents of $\ce{OH-}$ needed for the titration of a weak acid. For example here is the titration curve for Aspartate:

Why is the equivalents for $\mathrm{p}K_\mathrm{a1} = 0.5$? Why not $0.75$ or $0.80$? How was the answer 0.5 derived. How were the equivalents for $\mathrm{p}K_\mathrm{a2} = 1.5$? derived? I understand the $\mathrm{pI}$ is 1 because there's a balance of charges but why the 0.5, 1.5, and 2.5