The above graph is lifted from a forthcoming book by Surjit S. Bhalla, called Second Among Equals: The Middle Class Kingdoms of India and China. It shows the share of the middle class in world population rising from 2 percent in 1820 to 23 percent in 1950 to 54 percent in 2005 to a projected 79 percent in 2025. [typos corrected]
In another chart, looking at India alone, the middle class population is 2 percent in 1960 and 34 percent in 2005. China went from essentially 0 percent in 1980 to 67 percent in 2005.
Thanks to reader Prashant Kothari for forwarding the charts.