According to the paper (page 13), they only look at incomes between the 10th and 90th percentiles. That's why they cut off at $128,000 for US residents.

With that said, I do think it'd be interesting to see a graph that continues past the 90th percentile.

The paper: http://www.nber.org/papers/w18992?utm_campaign=ntw&utm_mediu...