“The only worse design than a pie chart is several of them.”—Edward R. Tufte, "The Visual Display of Quantitative Information"
The next time you need to put together a presentation on market share, media mix, or the latest market research survey, your first thought may be to create a pie chart when representing values that add up to 100 percent. Pie charts are ubiquitous in today’s media and even pop culture, but they happen to be one of the worst ways to visualize data.
Pie: Great for Dessert, Not for Data
Pie charts are only useful for very simple messages, such as, “Wow, that one slice is really big!” However, once you get past two or three slices, it becomes difficult to extract useful information from the chart. For example, in the chart below, it’s difficult to guess at the values of all but the largest slice, and it’s harder still to see how much larger any slice is than any other one.
The problem is that the human eye isn’t adept at sizing up angles, and especially not comparing different angles when they start in different places. We're also not good at connecting those pieces of the pie to actual numbers. To extract accurate information from the chart, you have to place data labels next to the slices. This raises the question: If you need to print the data values on the chart to make it useful (essentially, to make it a table), why generate a chart at all?
Bar Charts: The Sensible Choice
The remedy for all this confusion is to plot the data on a friendly bar (horizontal) or column (vertical) chart. You can easily see how the data from the pie chart above that brought on so much consternation is now much easier to read in bar chart format:
Now, it’s immediately clear how much market share each of the hospitals in the area command, because all of the bars are read off a single axis. This removes the need to label the individual bars. What’s more, you can easily tell that Hospital B’s market share is 5 percentage points larger than that of Hospital C. Bar charts are built with the human visual cortex in mind by playing to its strengths (which line is longer) instead of its weaknesses (which angle is wider).
Bar and column charts have many other benefits, including:
- You can add categories without reducing the clarity of existing categories.
- You don’t need to use different colors or a legend (save that color ink!).
- You aren’t restricted to types of data that add up to 100 percent or that include only positive values.
- You can add a series to easily compare categories over time, like the chart below.
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