This is a log-scale graph, which is what you want in order to compare rates of increase. You can see sort of an inflection point around the early 80s, but I'm not sure if it's significant.
Does this take into account inflation? It could be that they are actually decreasing with inflation. I don't think it's like that but I'm just curious.
That chart shows per capita income without adjusting for inflation. It's not very useful for drawing conclusions about real income.
And per capita is a simple average, while looking at medians or quintiles gives you a clearer picture. Per capita, a roomful of middle class workers all become billionaires if Bill Gates walks in.
Individual wages have risen steadily since 1910, with no gaps. The household income chart is a misleading chart.