Hierarchical data. As easy as pie.

A sunburst is a hierarchical pie chart.

Wikipedia tells us:

Sunburst chart or a multilevel pie chart, is used to visualize hierarchical data, depicted by concentric circles. The circle in the centre represents the root node, with the hierarchy moving outward from the center. A segment of the inner circle bears a hierarchical relationship to those segments of the outer circle which lie within the angular sweep of the parent segment.

Sunburst vs Treemap

Sunburst is often compared to the treemap visualization developed by Ben Shneiderman and Brian Johnson at the University of Maryland. Treemaps use a rectangular, space-filling slice-and-dice technique to visualize objects in the different levels of a hierarchy. Sunburst is an alternative to the treemap that uses a radial layout. The inner circle is the begining of the hierarchy and deeper levels are layered around this node.

Below are the same data displayed in a sunburst and a treemap. It displays Muhammad Ali's number of fights by win, loss and type of victory.


When to use Treemap vs Sunburst?

Treemaps are more efficient space-wise, in that they don't involve the same empty space at the center of the visualization that the Sunburst does.

Sunburst is more immediate. Most people are trained to understand a pie chart and a sunburst is just an evolution of a base pie chart. In 'An evaluation of space-filling information visualizations' authors demonstrate that treemap and sunburst performance in terms of time and accuracy of completion of a task are roughly equivalent. Nevertheless, most participants of the study chose the sunburst over the treemap.

Pie charts are often critized by data visualization specialists. Stephen Few, to name just one, advocates that a Pie chart should be replaced most of the time by bar/column charts.

"Visual perception supports only rough comparisons of areas and angles, which are the two primary ways that pie charts encode quantities."

Nevertheless, Stephen Few also mentions a strange fascination from human beings 'with all things circular'. There is aesthetic beauty in circular forms. A circle is a perfect form. What circles lose in precision they'll gain in intuitiveness. Sunbursts are beautiful and producing beautiful visualizations increases the attractivness and the engagement of your users. So even if you are a data visualization purist give the sunburst a try.