I think it's great exercise, but why don't they just use an absolute scale for parent's income? "About $2,400 in annual income separates the bottom two dots, while nearly $1 million separates the top two", I would never have guessed this either.
I felt strongly about this the moment I read the article on NYTimes - I reproduced the data set to the best extent I could and actually went about plotting against absolute income:
https://docs.google.com/spreadsheets/d/1jD87QN3pPZjoefrAm4E_...
That's a far more intuitive chart, that a lot of us will understand better. I also graphed how much these children actually go on to earn against absolute income. I compared these results in a blog post - http://unside.t4you.in/data/intuitive-axes/
HTH.