Before we go too far off the plumber deep end, consider there are more lawyers in the U.S. than plumbers:

http://m.wolframalpha.com/input/?i=plumbers+lawyers (note difference in salary as well)

Anyway, these numbers work best when you compare "all college graduates" to the "holy grail of all trades," the oft-referenced plumber. Most non-college graduates are not swimming in thr rich plumber mondy. Once you take out the bottom earners out of the college graduate pool, the difference in numbers becomes much more drastic, I think.