What's Wrong with using Pie Chart?

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. The earliest known pie chart is generally credited to William Playfair's Statistical Breviary of 1801.

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, box plot or dot plots.

An obvious flaw exhibited by pie charts is that they cannot show more than a few values without separating the visual encoding (the “slices”) from the data they represent (typically percentages). When slices become too small, pie charts have to rely on colors, textures or arrows so the reader can understand them. This makes them unsuitable for use with larger amounts of data.

Take a look at the very simple illustration above where just 6 data points are displayed on a pie chart and a bar graph.  The main goal of the visualization should be to relay the information to the viewer in an effective and precise manner.  

There is a few cases where pie chart can still function as a great visualization.  One example is when it is created to show components of something such as market share when there are only a handful of competitors exist.

As you can see from this example above, the pie chart here does its job pretty well.  However, various types of bar charts could have been used to create visuals that are just as effective.