I think the actual wall that is being built is between the Haves and the Have Nots, between the rich and the poor, between us and them. How better to conquer than to divide.
The graph shows an individual's contribution to the aggregate income of everyone in the USA and how the lower 50% of earners are contributing less now than in the recent past. Those lower income earners better start getting on the ball. The graph doesn't mean much more than that. If one is concerned about the income of the lower 50% of earners, then a more important graph would show how much their income has changed over that period, adjusted for inflation.
History teaches us that when that divide becomes to large there will be a revolution of some kind. I hate to think it would happen in the USA but if things do not come around I do not see any reason why our people would act any different!