By David Mendoza - Monday, March 16, 2015

Despite the overwhelming evidence proving pie charts ineffectively display data, designers continue to use this deficient graphic. Two of the most prominent data visualization experts, Stephen Few and Edward Tufte, both agree that the usefulness of the pie chart is limited. “Of all the graphs that play major roles in the lexicon of quantitative communication,” Few maintains, “the pie chart is by far the least effective.” Edward Tufte is even more blunt. In The Visual Display of Quantitative Information, he wrote, “Given their low data-density and failure to order numbers along a visual dimension, pie charts should never be used.”

Chart via reddit

And yet we continue to find pie charts everywhere. Recently, on /r/dataisbeautiful, this pie chart made it to the front page. The chart has several deficiencies, including the desaturated and nearly monochromatic color scheme, but its biggest flaw is its use of the 3D option. As flawed as pie charts already are, the use of an unnecessary third dimension makes its problems substantially worse.

Below I reveal exactly how much using 3D distorts the data displayed in the chart. On the left, I modified the original pie chart by increasing the color contrast to make the slices easier to differentiate. On the right, I created a 2D pie chart that more accurately displays the same data. I labeled the angle of each slice in blue. As the annotations show, both charts are not the same. For instance, the angle of slice 4 on the 3D pie chart should be almost twice as big, while slice 7 has the opposite problem. The angle of slice 7 should be around 50% smaller than it actually is.

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The distortion of the angles shown in the 3D pie chart also distorts the numbers underlying the visualization. Again, slice 4 and 7 are both incredibly inaccurate. Slice 4 should be 13.4%, but the actual value shown is 6.9%. To put that another way, slice 4 represents the 162 aircraft fatalities caused by the crash of Air Asia QZ8501, but on the 3D pie chart it really shows an angle equivalent to only 83 fatalities. Slice 7, on the other hand, should be 4%, but actually displays a value of 7.7%. That means instead of the 49 fatalities it’s supposed to represent it actually shows 93 fatalities — 47% larger than it should be.

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I decided to redesign the original chart by transforming it into a stacked bar chart. Since the original designer intended to show that 44% of aircraft fatalities were attributable to Malaysia Airlines, I grouped those two segments next to one another. This redesign does a more effective job displaying this data because people are better able to compare lengths than they can angles.

Click here to embiggen this image.

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