It seems like their sample population is too small to be fully confident in their conclusions. For example, why does the average salary for Java, Ruby, and Python specialists peak 3-5 years into their career and then decline? Why do early career candidates from top schools make less than their counterparts from worse schools? I don't think this can all be explained away with the equity argument and if it can, it probably makes the whole exercise moot.
What happen to iOS Engineers? I don't believe this infograph 100%.