When would you use a pie chart?
 Pie charts have become one of the most recognizable and widely adopted chart types in business and data journalism. Being that the message that pie charts convey is almost universally understood, it is tempting to incorporate pie charts into your own data visualization. However – data visualizer beware – while the message that pie charts convey is immediately recognized (parts of a whole), the insights that pie charts provide are not. Study after study has shown that pie charts are not the most effective means of communicating data. The pie chart’s primary limitation is that people are much better at comparing lengths and heights, as you would see in a bar or line chart, than they are at comparing areas within a pie. Further, the long tail results, or the thinner pieces of a pie, tend to become unreadable. Pie charts are also a very poor vehicle for communicating changes over time. Despite the empirical evidence against pie charts, we continue to see them in some of the world’s most trusted news sources, stock reports, and corporate dashboards, among many other places. It is easy to make fun of people’s use of pie charts (like this one!), but we want to be clear that when you hear people recommend avoiding pie charts, it is not some kind of elitist-data-visualization-specialist credence – it is a best practice based in evidence. We’ve always viewed the prolific use of pie charts as a huge opportunity to educate and improve the data visualization space. While graphical methods of statistics date back over 200 years, we still have a lot to improve – which is a great thing for you if you are reading this and are interested in evangelizing the benefits of data visualization. Bye Bye, PieTo help, here is a simple exercise to illustrate the reduction in time to insight from a pie chart to more appropriate chart types. First, have a look at this series of pie charts, showing the sales by department over a four-year period. Take 15-30 seconds and think of a couple of insights that the charts provide: You likely gathered that the entire pie was getting slightly larger over time (though it was hard to tell by how much and what that means), but it is pretty difficult to ascertain which pieces of the pie are causing the growth. With an easy tweak in Tableau, look at the exact same data as a line chart. Take the same 15 – 30 seconds to understand what the data is telling you – though it likely won’t take you nearly that long. Much easier, right? By converting our pie chart to a line chart, we can easily see that all three categories are on the rise, and by how much. We were able to process the visual and get to the insight much faster. If you are not as interested in the performance over time, and want to see how each category performed compared to each other each year, use a bar chart: Again, your brain processes heights of bars more easily than it can process areas in a pie. Using a bar chart also provides you more real estate to display the values, either as dollar amounts and/or percents of the whole (as you were trying to convey by using a pie chart). To take this one final step further, if you only care about comparing how each category performed against the others for one period in time, you can isolate the bar chart to show just the year in question: Just Can’t Let Go?We understand that you may have a stakeholder that has a hard time letting go of pie charts. If you or a boss can’t quite go cold turkey, here are a couple of tips for using pie charts while you transition away from them:
A pie chart helps organize and show data as a percentage of a whole. True to the name, this kind of visualization uses a circle to
represent the whole, and slices of that circle, or “pie”, to represent the specific categories that compose the whole. This type of chart helps the user compare the relationship between different dimensions (Ex. categories, products, individuals, countries, etc.) within a specific context. Usually, the chart splits the numerical data (measure) into percentages of the total sum. Each slice represents the proportion of the value, and should be measured accordingly. How do I read a Pie Chart?To read a pie chart, you must consider the area, arc length, and angle of every slice. Because it can be hard to compare the slices, meaningful organization is key. Slices in a pie chart should be organized in a coherent way, usually the biggest to smallest, to make it easier for the user to interpret. Start at the biggest piece and work your way down to the smallest to properly digest the data. The colors of the slices should match their respective blocks in the legend, so that viewers do not need to consult the legend as much. Generally, the whole (or total of the quantitative values or slices) is not listed within the pie chart. Typically, it can be listed in the text near the chart, the table explaining specific data measurements, or as a separate BAN in another dashboard. Three-dimensional pie charts are difficult to read and misleading. Since the interpretation of a pie chart relies on the area, arc length, and angle of each slice, a three-dimensional representation of this chart type will tilt the pie and skew your visual perception of its slices.The use of a three-dimensional pie chart can make an accurate comparison of categories nearly impossible due to the visual imbalance that can exist between slices. What type of analysis do Pie Charts support?Pie charts should be used to show the relationship of different parts to the whole. They work best with dimensions that have a limited number of categories. If you need to emphasize that one section of the whole is overrepresented or underrepresented, a pie chart can make that story within the data stand out. Pie charts do not work well for comparing exact numbers When and how to use Pie Charts for Visual AnalysisIf you have a dimension with just a couple of categories to compare, then a pie chart can help display each value of a category within the whole. The chart should read as a comparison of each group to each other, forming a whole category. The “whole” could be anything so long as the category can be split into separate slices that are distinguishable from each other. Use a pie chart if:
Do not use a pie chart if:
Pie Chart Best Practices:
If there is a comparison to be made between multiple categories, consider a line chart instead. Line charts provide an easy overview of the patterns and trends found in multiple sets of data, as well as the effect they have on each other. Great examples of Pie ChartsThis pie chart shows the percentage of total sales per regional market.
This pie chart takes data from the east market sales and splits them by the variety of coffee purchased.
Ineffective examples of Pie Charts and AlternativesPOOR EXAMPLEIn attempting to display the many kinds of coffee and tea sold at the market, someone created a very confusing pie chart.
BETTER ALTERNATIVEThe best way to present this kind of data would be through a bar chart. Bar charts provide space between categories, so that each bar is easily distinguished. Furthermore, each bar is aligned to a common baseline, the labels can be read more easily, and the relative size of each bar can be more easily determined by the viewer. POOR EXAMPLEIn attempting to display the many kinds of coffee and tea sold at the market, someone created a very confusing pie chart. Two or more pie charts should not be placed side by side for a comparison. Multiple markets show different percentages for the kinds of products sold.
BETTER ALTERNATIVEInstead, a stacked bar chart will make a better visual aide to compare multiple categories within a dimension. Stacked bar charts can also display parts of a whole through percentages. When the parts of the whole equal 100 percent of the total, users can better grasp the comparisons between categories. Further considerations and supplemental materialBar charts and stacked bar charts can be good potential alternatives to pie charts. They can better show the relative size and importance of measure values broken down by the categories of a selected dimension. They are also better equipped to show more categories at one time than a pie chart. When should a pie chart be used?There are two primary use cases for a pie chart: If you want your audience to have a general sense of the part-to-whole relationship in your data and comparing the precise sizes of the slices is less important. To convey that one segment of the total is relatively small or large.
What is pie chart and when it is used?Pie charts are often used to represent sample data—with data points belonging to a combination of different categories. Each of these categories is represented as a “slice of the pie.” The size of each slice is directly proportional to the number of data points that belong to a particular category.
When should you not use a pie chart?If you still feel the urge to use them, make sure you only use them for a percentage breakdown where each slice represents a certain percentage out of 100% and order the slices in size to make it easier to read. Never use a pie chart if it has more than 5 slices and never-ever make it 3D.
When would you use a pie chart instead of a bar graph?In short, a pie chart can only be used if the sum of the individual parts add up to a meaningful whole, and is built for visualizing how each part contributes to that whole. Meanwhile, a bar chart can be used for a broader range of data types, not just for breaking down a whole into components.
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