The chart’s title pretty clearly indicates the intended answer to Andrew’s question (which class is most charitable?): the poor.
I might, however, have included the somewhat relevant figure of actual average amount given to charity by each income group (calculated as average income times giving as percent of income):
Each richer income group appears to give more to charity than the next poorer group. The rich give the most to charity, and the poor give the least.
If someone truly cares about charitable giving, they ought to desire that as many people as possible should move into the “less generous” groups. Fetishizing the poor as the “most generous” group is a rather banal demonstration of the too-human tendency to confuse intentions with results – when results are clearly the desired metric.