Pie charts are circular charts divided into sections, used to illustrate percentages. In a pie chart, the whole circle represents 100%, and each part of the pie stands for a portion of that 100%.
This allows the viewer to see how something is divided up among different categories -- ideally less than seven categories, if you want your pie chart to be understandable.
The earliest pie chart is once again credited to our old friend William Playfair, the Scottish engineer, political economist and founder of graphical methods of statistics. Two pie charts appeared in his 1801 book Statistical Breviary, although they didn’t catch on at first; the next known use wasn’t until 1858, by French engineer Charles Joseph Minard.
Today, pie charts are possibly the most popular graphical chart, and you’ll see them pop up all over the place in the business world and in newspapers or TV. But it’s important to be aware that pie charts can be misrepresented, if it’s not clear what the whole stands for or if any of the parts are left out of the whole.
It can also be quite difficult to compare different sections of the pie chart, so pie charts are best used to compare the size of a particular slice with the whole, rather than comparing individual sections of the pie chart with each other. For comparing different categories of data against other categories, a bar chart may be more appropriate.
It’s also important to remember that people aren’t very good at reading angles or guessing area, so number labels should always be shown on the pie chart, as well.
3D pie charts are some of the worst offenders in bad visualization. Since angle is what encodes data in a pie chart, converting them to 3D distorts the very thing that displays the data.
Despite all these warnings, pie charts can be very powerful charts. They’re easy to read for some tasks, they’re familiar, and they look nice. Just avoid too many categories, and follow the tips above, and your pie chart should be fine.