His data appears to be Zipfian:
https://en.wikipedia.org/wiki/Zipf%27s_law
In this context, it's normal to graph with the x axis that is the "index in sorted list" as the author has done, although somewhat more common to make the y axis logarithmic.
https://en.wikipedia.org/wiki/Zipf%27s_law
In this context, it's normal to graph with the x axis that is the "index in sorted list" as the author has done, although somewhat more common to make the y axis logarithmic.