The plot above compares the COVID-19 new case growth rates over the past two months. The data are from The New York Times, based on reports from state and local health agencies^{[1]}.
The arrows are proportional to the number of new cases per 100,000 population. The latest number of new cases per 100,000 for the U.S. is given in the footnote.
You can estimate the new case rate for a state by camparing the arrow length with the National arrow (the thick black arrow). Specifics of the
state new case rates can be found by clicking the New Cases per 100,000 link above.

Arrows pointing to the left represent decreasing new cases in the current month. Arrows pointing down represent decreasing new cases last month. Arrows pointing left and down, then, represent decreasing new cases over the past two months, and arrows pointing right and up represent increasing new cases over both of the last two months.

The axes are labeled in qualitative terms describing the trends over these past two months. The actual angle of each arrow is precisely the ratio of the two months: an arrow pointing more to the side then the top or bottom indicates a bigger change in the last month than in the previous month.

[1]The New York Times. (2020). Coronavirus (Covid-19) Data in the United States. https://github.com/nytimes/covid-19-data