That graph is terrible and hard to decipher: from it alone I would not be able to say that there's any indication of a significant increase in real income for the poor.
OK that's a fair point. But suppose instead I said, "the percentage of the population living on less than $50k per year in real terms has dropped by about a quarter since 1971". Does the graph support that statement?
If you do use the graph as reference, it is actually more like the percentage dropped by ~ 11.5% (this is by counting out the cumulative percentage represented by each bar)- which is not at all what you would eyeball it at. Like I said, really bad graph to showcase anything.
The decline of the middle class and a new developed world of low wage working slaves straight out of Dickens / Hugo?