Here's a link to NBER data base. If you can find the source of the data that colloberates the assertion in the cartoon, please post in the comments. I don't doubt that the assertion is true, however, I could not find the study.
The Gini Coefficient measures the extent of income inequality. It is calculated by dividing the area between the line of perfect equality and the Lorenze curve by .5. Wikipedia lists the Gini Coefficient as .45. This area would be approximately .225. I just don't believe that this area is the same as it was in 1920.