There’s a doubly good discussion of the graph of falling incomes at Numbers Rule Your World.
First: that tracking the median income only of full-time workers masks the impact of changed work patterns. The proportion of people who can find full-time year-round work is smaller now than 30 years ago.
Second: that the median only goes down when the higher incomes decrease. If those in the lower half earn less, it has no impact on the median. They were below the mid-point and stayed below it. For the midpoint to fall, the income of some in the top half has to fall. For big drops to occur in the median, a lot of people in the top half must be getting reduced incomes.
The two effects take together — fewer full time jobs and drops I the median income — paint the picture of falling middle class incomes.