It's income rank, rather than actual income. In my explanatory text they mention...

jacalata on June 20, 2015 | parent | context | favorite | on: You Draw It: Family Income vs. College Attendence

It's income rank, rather than actual income. In my explanatory text they mention that the difference between two points on the far left is a few hundred dollars and the difference between two points on the right is a million or so (I forget the actual numbers).