The source is Delphi COVIDCast, Carnegie Melon University, which has been providing valuable and useful data and information since the pandemic started, that nearly every media site has been using as a primary source on this pandemic. This particular chart was printed by the Washington Post.
Here is there website: https://covidcast.cmu.edu/?sensor=do...w®ion=42003
It’s a serious chart and should be taken seriously.