Bubble Chart - Choosing Bubble Sizes Correctly

Choosing Bubble Sizes Correctly

The human visual system naturally experiences a disk's size in terms of its area. And the area of a disk—unlike its diameter or circumference—is not proportional to its radius, but to the square of the radius. So if one chooses to scale the disks' radii to the third data values directly, then the apparent size differences among the disks will be non-linear and misleading. To get a properly weighted scale, one must scale each disk's radius to the square root of the corresponding data value v₃ or scale each disk's circumference to the corresponding data value v₃ by using the straight linear relationship between the circumference and the radius:

where c is the circumference, r is the radius, and the Greek letter π is defined as the ratio of the circumference of the circle to its diameter.

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. So it is important that bubble charts not only be scaled in this way, but also be clearly labeled to document that it is area, rather than radius or diameter, that conveys the data.