Wednesday, July 17, 2019

Keep those legends

There was a debate today around maps from a Washington Post investigation about the opioid epidemic.

Some people praised them because they lack legends (other maps in the same story do have them). They argued that legends aren't sometimes that necessary because most viewers don't look at them anyway, as they just focus on the overall more-less message of the chart, not on its details. An article used in the debate even suggests removing sources.

I'm writing this on my phone, as I'm not in front of my computer today, so allow me to be brief: please never remove sources, and consider keeping your legends, particularly if you can make them unobtrusive (small, or placed behind an on/off button) and simple. Think twice before getting rid of them.

Legends aren't optional add-ons. They are an integral part of visualizations. I agree that a legend in a graphic aimed at a general audience shouldn't be overly detailed, but removing it completely may be going too far in most cases (I'll concede, though, that it might be acceptable in certain specific situations, including the map in the discussion, or these).

It turns out that I'm reviewing the results of some controlled observations I was part of, and I've seen that (a) many viewers can and do ignore legends but, (b) some others refer to them when reading a visualization, and feel frustrated and distrust the graphic if they can't find what they need. Contrary to what some of you may believe, this isn't dependent on education: people in this latter group didn't necessarily have college degrees. If you can place a legend on the side or at the bottom of your graphic, why wouldn't you serve them? Let's be cautious.