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bubble chart
A bubble chart is a type of chart that displays three dimensions of data. Each entity with its triplet (''v''1, ''v''2, ''v''3) 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.".〔(Present your data in a bubble chart ) Microsoft Office Online. Accessed 16 August, 2015.〕
== 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 ''v''3
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.〔Edward Tufte, ''The Visual Display of Quantitative Information''. Cheshire, CT: Graphics Press. (2001, 2nd edition, ISBN 0-9613921-4-2)〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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