No, and it is important to realize that the axes are project/tag count rankings not direct project/tag counts.

So, no conclusions regarding relative popularity between two languages should be drawn from the diagram. (It's just an ordering. The distance between the first and second most popular language, for example, is exactly 1.)

No. Original dataset from the site: http://www.dataists.com/wp-content/uploads/2010/12/language_...

[EDIT] It is surprising it's not exponential.