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Why do bubble charts make you misunderstand data size? Uncover the secrets of visual errors!
A bubble chart is a chart that displays three-dimensional relationships in data. Each entity is represented by three values (v1, v2, v3). These data are displayed in the shape of a disk. The position of the disk represents the first two values, and the size is used to express the third value. Such diagrams help viewers understand social, economic, medical, and other scientific relationships. However, despite their popularity in the data visualization world, bubble charts can lead viewers to misunderstand the true size of the data.
Correct selection of bubble size
Using bubbles to display scalar (one-dimensional) values can often be misleading. The human visual system tends to perceive the size of a disk based on its diameter rather than its area. Therefore, most charting software requires the user to provide the radius or diameter of the bubble as a third value. Such scaling patterns can lead to serious misunderstandings, especially when the data range is large. Since many people are not familiar with this, or do not stop to consider the implications of this issue, even those who are aware of this issue often have to hesitate when interpreting bubble plots because they cannot assume scaling corrections. It has indeed been adopted.
Bubble charts not only need to be scaled correctly, but they must also be clearly labeled to inform readers that the data being represented is area, not radius or diameter.
Display of zero or negative data values
The metaphorical representation of data values as the area of a disk cannot be used to display negative or zero values. In many cases, users of bubble charts use graphical symbols to represent non-positive data values. For example, negative values can be represented by a disc with a sign in the center to show that the size of the disc represents the absolute value of the negative value. Additionally, for zero values, some users choose not to use a disk at all, but instead use a square to represent the corresponding position, while others use filled circles for positive values and open circles for negative values.
Incorporate further data dimensions
In addition to the three main values, bubble charts can also incorporate additional information about the entities through systematic selection of the color and style of the disks. In addition, text annotations can be added to the discs, and simple labels can be used to cross-reference related instructions.
Other uses
In the field of architecture, a "bubble diagram" also refers to a preliminary architectural design sketch composed of bubbles. In software engineering, a bubble diagram can refer to a data flow, data structure, or other diagram that uses circles or bubbles to show entities and their relationships. In information visualization, bubble charts may be used as a technique for representing a set of values, where the area of the circle is proportional to the value. Unlike traditional bubble charts, this type of display does not assign meaning to x- or y-axis positions, but instead attempts to pack circles tightly to use space more efficiently. This method was introduced by Fernanda Viegas and Martin Wattenberg and quickly became a popular way of displaying data. Circle packed charts, included in popular visualization tool packages such as D3, have also been used by The New York Times.
In this context, have you ever made incorrect judgments on data due to misunderstandings of bubble charts?
