I believe they plot the annual increase in absolute numbers. Simply Δf=f(t1)-f(t0), where t1-t0=1[year]. It is not very sophisticated mathematically, but it is informative, in the sense that somebody looking for a job would be interested in the absolute dynamics Δf, perhaps more than in the absolute values f, and certainly more than in any relative values such as Δf/f(0). Let's go where they need X, not where they already have X.

Also, it is informative for forming a general take on the working America - I already know that in Texas there are cowboys and in Cali there are beach-tanned girls, but I'd like to adjust my stereotypes with time, not to be stuck in the sixties or whatever. I'm honestly impressed not just with South Dakota welcoming all those herds of software engineers, but with almost every state growing with some usefully busy people.