The graph is in cases per million as the states vary in population from 1.9 million (NE) to 19.5 million (NY). Note that 100,000 cases per million translates to 10% of the state's population if we assume each resident can only be infected by COVID- 19 once.
I believe this follows Edward Tufte's intent and style more than graphs shown in the article. It also covers a longer time period.
Noticing the higher cases in Nebraska and Iowa would naturally lead to investigate neighboring "Plain States" and finding that North and South Dakota are even higher in cases per million.
For the full article Dodging Deceptive Depictions - https://asq.org/quality-progress/articles/dodging-deceptive-depictions?id=bfb9d2c64f8c4f2d9f4ceca0ec37796d