1300 samples? With JavaScript being the strongest skill across the board? I'm not sure I actually believe the word "senior" in any occurrence on this page.
Its seems contradictory that the roles requiring years of experience (or they ought to) have JavaScript as the top skill. I would have expected an older less trendy language (like Java or C, or SQL) to be top in those positions.
As for the sample size. 1373 profiles supplied job titles that fell into these buckets out of approximately 3500 at the time we generated the infographic.