A pie chart (or a circle chart) is a circular statistical graphic, which is divided into slices to illustrate numerical proportion. In a pie chart, the arc length of each slice (and consequently its central angle and area), is proportional to the quantity it represents. While it is named for its resemblance to a pie which has been sliced, there are variations on the way it can be presented.
Pie charts are perhaps the most ubiquitous chart type; they can be found in newspapers, business reports, and many other places. But few people actually understand the function of the pie chart and how to use it properly. In addition to issues stemming from using too many categories, the biggest problem is getting the basic premise: that the pie slices sum up to a meaningful whole.
Pie charts are very widely used in the business world and the mass media. However, they have been criticized, and many experts recommend avoiding them, pointing out that research has shown it is difficult to compare different sections of a given pie chart, or to compare data across different pie charts. Pie charts can be replaced in most cases by other plots such as the bar chart.
Pie charts are a visual way of displaying data that might otherwise be given in a small table. Pie charts are useful for displaying data that are classified into nominal or ordinal categories. Nominal data are categorized according to descriptive or qualitative information such as county of birth or type of pet owned. Ordinal data are similar but the different categories can also be ranked, for example in a survey people may be asked to say whether they classed something as very poor, poor, fair, good, very good.
Pie charts are generally used to show percentage or proportional data and usually the percentage represented by each category is provided next to the corresponding slice of pie.
Pie charts are good for displaying data for around 6 categories or fewer. When there are more categories it is difficult for the eye to distinguish between the relative sizes of the different sectors and so the chart becomes difficult to interpret.
The main use of a pie chart is to show comparison. When items are presented on a pie chart, you can easily see which item is the most popular and which is the least popular.
Various applications of pie charts can be found in business, school, and at home. For business, pie charts can be used to show the success or failure of certain products or services. It can be used to show market reach of a business compared to other comparable businesses.
At school, pie chart applications include showing how much time is allotted to each subject. It can also be used to show the number of girls to boys in various classes. At home, pie charts can be useful when figuring out your diet. You can also use pie charts to see how much money you spend in different areas.
There are many applications of pie charts and all are designed to help you to easily grasp a bunch of information visually.
There are some simple criteria that you can use to determine whether a pie chart is the right choice for your data.
Do the parts make up a meaningful whole? If not, use a different chart. Only use a pie chart if you can define the entire set in a way that makes sense to the viewer.
Are the parts mutually exclusive? If there is overlap between the parts, use a different chart.
Do you want to compare the parts to each other or the parts to the whole? If the main purpose is to compare between the parts, use a different chart. The main purpose of the pie chart is to show part-whole relationships.
How many parts do you have? If there are more than five to seven, use a different chart. Pie charts with lots of slices (or slices of very different size) are hard to read.
Care must be taken to retain the salient feature of the chart: the center. In a report on the number of words consumed each day, a pie chart was prettified with a highlight in the middle. That obscures the spot where the lines meet, and thus makes it impossible to judge angles, making the comparison more difficult.
This is similar to the problem with a colleague of the pie chart, the donut chart. It is similar to the pie chart, but is missing a circular area in the center.
While the center may be a convenient spot for labeling, it degrades the chart’s readability. The comparison between separate pie or donut charts is also largely meaningless, and should be avoided. To show progression over time, line and bar charts are much better suited. To compare two different kinds of data (absolute numbers and fractions), it makes more sense to split them up by data to compare than by year.
The most common problem is trying to show too many categories in a single pie chart. Wikipedia has this beautiful specimen on the page on U.S. states by population. The first four states are clearly larger than any of the rest, and from there the chart turns from a meaningful visualization of numbers into a colorful pattern.
A bar chart would have been a much better idea here, because it would have allowed easier comparison between the states. Grouping together states of similar size into separate charts with different scales would have made it possible to clearly see the differences for all of them, not just the most populous ones.