I think what this graph shows is it is really, really hard to break into the jobs at entry level without a college degree. If you fail to break in after a couple years of trying, I suspect most people give up and permanently have their income lowered as a result.
I really, really wish these stats were based on 'same profession'. [e.g. Software Developers w/ Degrees vs. Software Developers w/o Degrees]
And I highly suspect that the reason that is never done is it will show that the value of the degree vs. a guy who took $15/hr contracting jobs for a couple years until he 'broke in' to being a Software Dev is close to 0. At which point, the self taught dev is ahead in both 'work experience' and financially.
But I could be wrong, no one has done that sort of study that I know of :/
EDIT: The closest I could find is: http://cew.georgetown.edu/collegepayoff https://georgetown.app.box.com/s/ctg48m85ftqm7q1vex8y
It shows ~$700k lifetime earnings gap for STEM [bachelors vs. highschool diploma]. So, over ~45 years, we are talking a loss of ~15.5k/year on average.
Its not quite as precise as what I was hoping for but such is life since I wanted Position to Position.