In our Data Visualization 101 series, we cover each chart type to help you sharpen your data visualization skills.
For a general data refresher, start here.
Pie charts are one of the oldest and most popular ways to visualize data. This classic chart is the perfect example of the power of data visualization: a simple, easy-to-understand presentation that helps readers instantly identify the parts of a whole. Without further ado, here’s everything you need to know about the pie chart.
What It Is
The typical pie chart is divided into sections that illustrate a numerical proportion. Each section expresses its quantity through the size of the central angle in proportion to the others. What makes this visually powerful is that the central angle, the outside arc length, and the area these define all proportionally correspond to the quantity they represent. Isn’t geometry great?
Where It Came From
Scottish engineer William Playfair is generally credited with creating the world’s first pie charts back in 1801. The chart was featured in his now-famous work, “The Statistical Breviary.” Almost two centuries after his death, Playfair’s pie chart remains fundamentally the same.
Playfair’s pie chart showed the proportion of land held by the Turkish Empire in Asia, Europe and Africa.
Although he invented the form, Playfair never called his invention a “pie chart.” It was left for later generations to note the dessert’s similarity. It also wasn’t the only food-based metaphor used to describe the chart. The French casually refer to them as “Camembert” after their delicious round cheese.
Another series of Playfair’s pie charts in “Chart Representing the Extent, Population & Revenue of the Principal Nations in Europe in 1804.”
After Playfair’s initial work, the pie chart began to pick up steam. In 1858, French engineer Charles Joseph Minard used pie charts to show the cattle sent from different regions of France for consumption in Paris.
When to Use a Pie Chart
Pie charts are best used when making part-to-whole comparisons (for example, the number of red cars produced each year compared to other popular colors). The message that we are observing fractions of a whole amount is built right into their design—something that can’t be said for most other chart types. They have the most impact when the proportion being expressed holds more importance than the specific numbers. They are most clearly understood when using small data sets, often grouping smaller data into an “other” category on the chart.
Note: Not everyone is a fan of the pie chart, however. Critics like Steven Few argue that we have trouble distinguishing proportion in the segments, meaning the impact of the data is lost or misinterpreted. In this case, they make the argument that it is better to use a bar chart when specific numbers have importance or large data sets are a factor.
Tips for Creating Pie Charts
Pie charts are very popular, but they are often poorly designed (another reason critics don’t like them). When creating your pie charts, make sure to follow these best practices.
1. Visualize no more than 5 categories per chart.
It is difficult to differentiate between small values; depicting too many slices decreases the impact of the visualization. If needed, you can group smaller values into an “other” or “miscellaneous” category, but make sure it does not hide interesting or significant information.
2. Don’t use multiple pie charts for comparison.
Slice sizes are very difficult to compare side-by-side. Use a stacked bar chart instead.
3. Make sure all data adds up to 100%.
Verify that values total 100% and that pie slices are sized proportionately to their corresponding value.
4. Order slices correctly.
There are two ways to order sections, both of which are meant to aid comprehension:
Option 1: Place the largest section at 12 o’clock, going clockwise. Place the second largest section at 12 o’clock, going counterclockwise. The remaining sections can be placed below, continuing counterclockwise.
Option 2: Start the largest section at 12 o’clock, going clockwise. Place remaining sections in descending order, going clockwise.
Stay tuned for our next installment of Data Visualization 101, where we will explore the linear goodness of the bar graph.
And for more on how to design your data, download our free e-book: