Introduction to area and volume charts

Area and volume charts can be used to communicate the magnitude of benefit and risk metrics by showing the areas or volumes of different sizes for the purpose of illustrating whether a difference exists between two entities.

Area graphs are commonly two-way graphs with an extra dimension represented by the size of the area. Volume charts add an extra dimension on top of the area graph represented by the depth. Area graphs and volume charts may also be presented without meaningful axes. An area graph may appear as a line graph but the more important information is represented by the area. In some cases, area graphs are used in place of line graphs, by adding colours or shades under the line, to communicate trends in data series, simply because of the added perceived attractiveness. The focus on attractiveness of graphics is also known as “infographics”.

The goals of infographics include displaying the information in a readily understandable way, telling a story, and attracting attention and stimulating interest. Infographics should ideally be attractive but may not necessarily be informative at the right level. [Gelman A, Unwin A. Infovis and statistical graphics: Different goals, different looks (with discussion). Journal of Computational and Graphical Statistics. In press 2012.]

Area graphs and volume charts suffer from human’s ability to perceive area and volume accurately. In the case of volume chart, it becomes worse because of our limitation to accurately judge the size of three-dimensional objects.

Area graphs can be generated in many software packages including Stata, R, SAS, SPSS, Tableau, Spotfire,QlikView, IBM Many Eyes, Microsoft Excel®, and Google Drive. The volume chart is more restrictive in terms of production since not all software packages support three-dimensional graphics, but R, SAS and Microsoft Excel are some packages that could be used. Area charts can be used interactively or dynamically to show how certain measures change, for example, over time but not exactly by how much they change. Dynamic and interactive area graphs could overcome some of the limitations faced when presenting static area graphs since better functionalities and guidance can be built into the graphics to aid understanding. Example applications of dynamic and interactive area graphs can be seen on the Gap Minder website, such as and Interactive volume chart may apply the same principles as area graphs but it may not have much added value in terms of helping users making decisions.