Uses of the Doughnut Chart
In terms of appearance and function, the doughnut chart is very similar to a pie chart. Both break down tables of data to give the viewer an idea of the percentage distribution that each item in the table represents. The most obvious difference in actual appearance of the two charts is that the doughnut chart has a "hole" in the middle – thus, explaining why it's referred to as a doughnut.
One big advantage that the doughnut chart has over the pie chart is that it allows you to graph multiple data series, so you can use the chart to show how a distribution changes when measured for different time periods, locations, or other factors. For example, in the chart shown to the left (click any image in this article for a larger view), we've used a doughnut chart to compare browser usage in January 2009 versus June 2009. The data for this chart comes from the monthly web statistics report at w3schools.com. By representing the data in this way, we can quickly see where any major shifts, if any, of web browser usage occurred.
Doughnut charts are best employed when the number of items in your table is on the low side. If you try to use them to compare too many things, the data quickly gets lost as the number of doughnut slices increases, especially if many of the slices are only very small percentages of the total collection. However, in this latter case, you can get a little better representation of your data if you use an exploded doughnut chart. We'll talk more about that in the next section.