Tuesday, November 27, 2012

Radar graphs: Avoid them (99.9% of the time)

Stephen Few doesn't like radar graphs, and he's not the only one who has written against them. In a recent discussion on Twitter, Jon Peltier said that they are "worse than pies" —ouch! Even Andy Kirk, who is usually as polite as a British gentleman can be, doesn't have nice words about this kind of display.

Most of the arguments against radar graphs can be summarized in a couple of sentences from this post by Graham Odds: "Even with a common scale between axes, comparing values across them remains cumbersome and error-prone. This is because rather than the simple straight-line comparison our visual perception is hard-wired to perform that is found in “conventional” chart types, comparison in radar charts requires conscious thought to mentally project a sort of arc of rotation to map a value from one axis onto another, something we are not particularly adept at."

I agree on that, something that you may find unsurprising if you have read my book already. However, I believe that there are some very specific situations in which radar graphs may be appropriate, hence the title of this post (disclaimer: I've used them just twice in my career.)

One of those cases is mentioned by Stephen Few himself at the end of his article. Another one can be illustrated with the infographic on the left, made by Matt Perry, head of graphics at the San Diego Union-Tribune. Matt sent me that page the other day, saying that he was loosely inspired by an example in The Functional Art. This one:

Época Magazine

In these two infographics, neighborhoods (Matt's piece) and Brazilian states (mine) are represented by radii arranged according to their geographical locations. We included maps in the compositions to provide some context.

The goal of both displays is to make a point: Votes for each party tend to concentrate in certain regions. In the latest Brazilian presidential election, the candidate of the Partido dos Trabalhadores (liberal: red line) won in the North East —Brazil's poorest area— by wide margins, but was tied with her main rival in other parts of the country. In San Diego, Republicans won in the northern neighborhoods, and Democrats were dominant in the southern ones.

I guess that we could argue that a parallel coordinates graph would make that message equally clear, an idea suggested by Lynn Cherny. You never know if a visual form will work until you give it a try, so I redesigned my radar graph this morning:

Not bad at all. It is certainly better if you want to accurately compare states with each other, but I have some concerns about it:

1. Is the original goal —to uncover general, regional voting patterns— as intuitively achieved as in the radar graph? I don't think so. To begin with, you need to read the labels at the bottom to identify each region. The radar graph works as a compass: North-eastern states are on the top-left; southern states are at the bottom. Besides, it is impossible to avoid certain odd features in the parallel coordinates graph: For instance, Mato Grosso (MT) and Tocantins (TO), two states that are side by side on the map, are in opposite extremes of the plot. Radar graphs look a bit more natural in these examples, as they somehow resemble the maps they are based on. Encoding the data in a parallel coordinates graph adds a needless layer of abstraction.

2. Most of the time, facilitating accurate comparisons and judgments must be our primary goal, but not always. If that were the case, we would never use choropleth maps, or proportional symbol maps, to mention just a couple of graphic forms which are adequate to reveal broad patterns. I'd recommend you to read Nathan Yau and the classic paper by Cleveland and McGill he discusses there.

Obviously, you should take my words with skepticism. This post is based on intuitions, and those rarely qualify as solid arguments, as one of greatest gems of epistemological wisdom suggests. I would love to hear your thoughts.