I believe the graph expresses CO2 emissions of each income level as a percentage of the total.

On a global scale it means that more poor people are starting to live in industrialized nations. As opposed to living on a farm without electricity.

It doesn't mean that the rich are producing less CO2. They can be producing more, but a larger part of the total is made by other income levels.

Also, given that the rich can hide their income with tax evading tricks, and that a lot more CO2 is produced by industry than individuals anyway, that whole graph isn't particularly useful.