Wednesday, May 20, 2020

About that weird Georgia chart

Visualization social media has been busy mocking the following chart by the Georgia Department of Public Health. Pay attention to its horizontal axis:

I never attribute malice when sloppiness is a more parsimonious explanation. I guess that whoever designed this chart thought that sorting the bar groups from highest to lowest, instead of chronologically, was a good idea.

This is not wrong per se; it's possible to think of situations when it's useful to arrange your data like this during analysis. As it always happens in visualization, design choices depend on purpose.

In this case, though, the purpose is to show “the most impacted counties over the past 15 days and the number of cases over time,” so separating the counties and then sorting their bars chronologically seems to make more sense. Something like this:

Visualization books, including mine, spend many pages discussing how to choose encodings to match the intended purpose of every graphic, but we pay too little attention to the nuances of sorting: should we do it alphabetically, by geographic unit, by time, from highest to lowest, from lowest to highest—or do we need an ad-hoc criterion? Or should we make the graphic interactive and let people choose? As always, the answer will depend on what we want the reader to get from the visualization.