Bubble Charts



Bubble charts are an ideal means of expressing data which are specified by three values (or attributes). An example might be age, income and weight, and two values are nominated to be represented on the x and y axis of the chart and the third value is represented by the bubble size.

There is a well know danger associated with bubble charts, and this relates to the manner in which a value is translated into the bubble. If the value is a direct map to the radius (or diameter) of a bubble then differences are effectively squared, since the area of a circle is pi times radius squared. This can be very misleading, particularly when the range of values is large. A more accurate representation is to use the square root of the value that is used for bubble size. It is not unknown for this unfair representation to be used deliberately to convey a desired message.

Representation of negative values can be problematical, since the area of a circle cannot have a negative value. This can be got around by using a different color for negative values (red probably) or marking negative values with a particular symbol.

Bubble charts are well represented in all charting tools including Microsoft Excel.