when the research and analysis firm Forrester recently surveyed our readers about how much time they spend writing in any given language
When the results change over time as indicated by the two charts, that can mean one of two things: either a lot of people who worked in just one language in 2010 now work in several languages, or a lot of people who work in just one language have stopped reading Dr. Dobbs. In order to support the claim that it's mainly the first and not the second possibility, one would have to submit at least some supporting evidence. (Edit: at first I thought that one would want to contact the same people in both surveys, but no, that's not good. It leaves out the effect of new people entering the arena. This doesn't seem to be easy.)
And also, this perhaps being a matter of taste, I find it questionable to have a caption like "Fraction of programmers..." underneath the charts. As much as I respect and admire Dr. Dobbs (I have published there): not every programmer is a Dr. Dobbs reader.