If you follow economic news, the chart shouldn't be all that surprising. India and China grew dramatically between 1988 and 2008, as did a number of smaller middle-income nations, accounting for the big spike in incomes for the global middle class. Meanwhile, top incomes in developed countries surged while median incomes stagnated, forming that Krugman calls the "valley of despond."
Figuring out how to fill in that valley is an important, and extremely difficult, challenge. But the more important task is figuring out how to boost growth of incomes for the world's poorest people. Their incomes grew comparatively less than those of the global middle class, according to Milanovic's numbers, and research by Georgetown's Martin Ravallion suggests that living standards for the world's absolute poorest people have risen less than most poverty metrics show.