Happy Pi Day everyone. You know: March 14, or 3.14. The puns will litter your Facebook feed, and, no doubt, your local bakery has already cooked up a one-day deal. But while pi is important, let’s take a moment to also honor the humble pie … chart.

Pi, a brief history

First, some context. Pi, the mathematical constant, has been with us for a while. The ratio of a circle’s circumference to its diameter, the Egyptians and Babylonians computed the famous number (placing it somewhere between to 3.125 and 3.16) and used it to engineer pyramids and ziggurats. And the Greek Archimedes was the first to derive pi mathematically to seven digits (using a compass and polygons, no less). Then in the 19th century, William Jones first used the greek notation π, probably as an abbreviation of ‘periphery.’

Pie, for data

But at Qualtrics, we’re much more enthused by ‘pie’ charts. And their invention is more recent than you might imagine. Scottish mathemetician William Playfair published his Statistical Breviary in 1801, basically an almanac that showed which portions of different empires fell in different continents. Playfair is credited as the first to use circular notation to show component parts in a visual whole. And, since then, the pie chart has made its way into grade school fractions, every beginner’s Excel report, and more than a few memes.

But just because pie charts are everywhere, doesn’t mean you should always use them. In fact, Edward Tufte, the godfather of data visualization, argues against using them at all.

When to use a pie chart

So when is the pie chart a good way to display your data? Here are three guidelines:

  • When you need to communicate a simple proportion of values that combine to a whole, or 100%.
  • When you need to convey information to a lay audience. Most people have been drilled on how to read a pie chart since grade school. So even if they don’t have a statistical background, you don’t have to spend much time explaining parts of a whole.
  • When you have fewer than six categories. Some experts argue for even fewer than six. But either way, limit your categories in the chart, and make sure they are clearly labeled and visually distinct.

In short, Happy Pi(e Chart) Day. Use them wisely.