Concept
A bubble chart is a type of chart that displays three dimensions of data. Each entity with its triplet (v1, v2, v3) of associated data is plotted as a disk that expresses two of the vi values through the disk's xy location and the third through its size. Bubble charts can facilitate the understanding of social, economical, medical, and other scientific relationships. Bubble charts can be considered a variation of the scatter plot, in which the data points are replaced with bubbles. As the documentation for Microsoft Office explains, "You can use a bubble chart instead of a scatter chart if your data has three data series that each contain a set of values. The sizes of the bubbles are determined by the values in the third data series.". Using bubbles to represent scalar (one-dimensional) values can be misleading. The human visual system most naturally experiences a disk's size in terms of its diameter, rather than area. This is why most charting software requests the radius or diameter of the bubble as the third data value (after horizontal and vertical axis data). Scaling the size of bubbles based on area can be misleading [ibid]. This scaling issue can lead to extreme misinterpretations, especially where the range of the data has a large spread. And because many people are unfamiliar with—or do not stop to consider—the issue and its impact on perception, those who are aware of it often have to hesitate in interpreting a bubble chart because they cannot assume that the scaling correction was indeed made. It is therefore important that bubble charts not only be scaled correctly, but also be clearly labeled to document that it is area, rather than radius or diameter, that conveys the data. Judgments based on bubble sizes can be problematic regardless of whether area or diameter is used. For example, bubble charts can lead to misinterpretations such as the weighted average illusion, where the sizes of bubbles are taken into account when estimating the mean x- and y-values of the scatterplot.
