George Mason University economics professor explains:
Along the horizontal axis are within-country income percentiles running from the bottom 5% (1st ventile) to the top 5% (20th ventile). Along the vertical axis are world income percentiles.
The graph shows that the bottom 5% of Brazilians are among the poorest people in the world but the top 5% are among the richest. Thus the vertical range of the curve tells us about within-country inequality.
Comparing between countries we see that the poorest 5% of Americans are among the richest people in the world (richer than nearly 70% of other people in the world). The poorest 5% of Americans, for example, are richer than the richest 5% of Indians.