Graduation Rates and Student income

There is an interesting piece on the New York Times (The Upshot) about the relationship between graduation rates and low income students.  The graph below is from the essay.  It relates graduation rates (six year rates) with the percentage of students receiving Pell grants.

The authors of the study discussed state:

Colleges that have high graduation rates tend to be more selective and tend to have students who are more affluent and more academically prepared. Colleges with lower graduation rates tend to admit a higher percentage of students with Pell grants, which usually go to lower-income students.