It's not a bad graph, it is just not familiar. Perhaps it would be more clear sticking to a single value of US GDP, e.g. as "% of world pop. above 1/4 of US GD

It's not a bad graph, it is just not familiar. Perhaps it would be more clear sticking to a single value of US GDP, e.g. as "% of world pop. above 1/4 of US GDP per person, over time", but the data shown only has three time points ... (Thanks for the pic, it was interesting.)