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From zero to negative numbers, how does a bubble chart display "invisible" data?
A bubble chart is a type of chart that displays data in three dimensions. Each entity is plotted with its data associated with a triple (v1, v2, v3) and is represented as a disk with two values represented by the disk's xy position and the third value represented by The size of the circle is expressed. The use of bubble charts facilitates understanding of social, economic, medical, and other scientific relationships. A bubble chart can be thought of as a variation of a scatter chart, where the data points are replaced by bubbles.
"If your data has three data series, each containing a set of values, you can use a bubble chart instead of a scatter chart."
Choosing the correct bubble size is critical because using bubbles to represent scalar (single-dimensional) values can be misleading. The human visual system most naturally perceives the size of a disk as its diameter, not its area. This is why most charting software requires the radius or diameter of the bubble as a third data value. Scaling bubble sizes based on area can lead to serious misinterpretations, especially when the data range is large.
This scaling problem has the potential to be extremely confusing, especially to those who are not familiar with the subject.
Therefore, bubble charts not only need to be scaled correctly, but also clearly labeled so people know that it is the area, not the radius or diameter, that is conveying the data. Judging the size of bubbles is problematic, whether using area or diameter. For example, bubble charts can give the illusion of a weighted average, where people take the size of the bubbles into account when estimating the average x- and y-values of a scatter plot. The bubble size range is often set arbitrarily, e.g. the maximum bubble size is often set to a small fraction of the total chart width and may not correspond to an actual measurement.
The Challenges of Displaying Zero or Negative Data Values
As a metaphorical representation of data values, bubble charts cannot be extended to show negative or zero values. In this case, some users of bubble charts choose to use graphical symbols to represent non-positive data. For example, when a value v is less than zero, a disk might be used with a chosen symbol such as "×" in the middle to indicate that the size of the bubble represents the absolute value of the negative number.
This approach is relatively effective when the magnitude (absolute value) of the data values is of some importance.
For data representing zero values, some users do not use a disk at all, but instead use a square in the corresponding position. Other methods may use solid circles for positive values and hollow circles for negative values. The systematic choice of colors and patterns on these charts can also convey a lot of further dimensions of data information.
Possibility of integrating more data dimensions
In addition to the three main values of the bubble, it is often possible to incorporate additional information about the entity through the use of different colors and patterns. It is also possible to annotate the discs, adding textual information which could be a unique identification tag to cross-reference explanatory keys and other categories.
Other uses of bubble charts
In architecture, the term "bubble diagram" is also used for layouts in first architectural sketches, where the shapes are shown as bubbles. In software engineering, a "bubble diagram" may refer to a data flow, data structure, or other diagram in which entities are represented by circles or bubbles and relationships are represented by connecting lines. In information visualization, a bubble chart may refer to a set of quantities represented by tightly packed circles whose areas are proportional to the quantities.
These bubble charts were first introduced by Fernanda Viegas and Martin Wattenberg and quickly became a popular method for displaying data.
Circle wrappers are widely used in popular visualization toolkits such as the D3 library and are used by The New York Times.
Conclusion
In the design of bubble charts, how to deal with seemingly "non-existent" data such as negative numbers and zero values is still a major challenge. In the meantime, what can the misconceptions about interpreting these charts teach us?
