You have two numbers +84% and -25%.
The textbook method to visualize this pair is to plot two bars. One bar in the positive direction, the other in the negative direction. The chart is clear (more on the analysis later).
But some find this graphic ugly. They don’t like straight lines, right angles and such. They prefer circles, and bends. Like PBS, who put out the following graphic that was forwarded to me by Fletcher D. on twitter:
Bending the columns is not as simple as it seems. Notice that the designer adds red arrows pointing up and down. Because the circle rounds onto itself, the sense of direction is lost. Now, readers must pick up the magnitude and the direction separately. It doesn’t help that zero is placed at the bottom of the circle.
Can we treat direction like we would on a bar chart? Make counter-clockwise the negative direction. This is what it looks like:
But it’s confusing. I made the PBS design worse because now, the value of each position on the circle depends on knowing whether the arrow points up or down. So, we couldn’t remove those red arrows.
The limitations of the “racetrack” design reveal themselves in similar data that are just a shade different. Here are a couple of scenarios to ponder:
- You have growth exceeding 100%. This is a hard problem.
- You have three or more rates to compare. Making one circle for each rate quickly becomes cluttered. You may make a course with multiple racetracks. But anyone who runs track can tell you the outside lanes are not the same distance as the inside. I wrote about this issue in a long-ago post (see here).
For a Trifecta Checkup (link), I'd also have concerns about the analytics. There are so many differences between the states that have required masks and states that haven't - the implied causality is far from proven by this simple comparison. For example, it would be interesting to see the variability around these averages - by state or even by county.