All metrics are imperfect: this one of course over-represents languages used by employers. It won't capture self-employment (a significant and growing trend). Also, it's uncomfortably common to advertise for one thing in order to filter applicants, when the job is actually for something else. (Maybe it's clever of them, but I've felt cheated by this bait-and-switch.) Along these lines, ability to pick up lesser skills is assumed. (Perhaps that's why PHP doesn't appear.)

Stackoverflow/web searches over-represent languages used with the web (esp Javascript). But, since the web is the fastest-growing industry, it may be an accurate anticipation.

You may want to correct your spelling of Clojure: http://www.indeed.com/q-Closure-jobs.html