He explains in the text: "The releases are distributed uniformly on the Y axis". In other words, there is an equal amount of space between each release.
Although someone does point out in the comments that he has left out the initial release of Tiger/x86, which was arguably a 0-features follow-on to Tiger/PPC. That would have changed the shape of the curve. The choice is completely arbitrary.
Although someone does point out in the comments that he has left out the initial release of Tiger/x86, which was arguably a 0-features follow-on to Tiger/PPC. That would have changed the shape of the curve. The choice is completely arbitrary.