What Are Bubble Charts and How Are They Used?
See also "How to Make a Bubble Chart" by clicking here.
Bubble charts or bubble graphs are extremely useful graphs for comparing the relationships between data objects in 3 numeric-data dimensions: the X-axis data, the Y-axis data, and data represented by the bubble size. Essentially, bubble charts are like XY scatter graphs except that each point on the scatter graph has an additional data value associated with it that is represented by the size of a circle or “bubble” centered around the XY point.
Bubble Chart Example 1: This chart shows the the relationship between “Profit” (Y-Axis), “Cost” (X-Axis), and “Probability of Success (%)” (Bubble Size).
Bubble Chart Example 2: The same as Example 1, but the bubble size variable has been removed so now it is displayed as a scatter chart.
Bubble Charts in Business
Bubble charts are often used in business to visualize the relationships between projects or investment alternatives in dimensions such as cost, value, and risk. By visualizing project portfolios using bubble charts, you can find clusters of relatively attractive projects in one area of the graph, such as areas of high value, low cost, and/or low risk, and compare them with relatively less attractive projects in a different area of the graph, such as an area of low value, high cost, and/or high risk.
In the bubble chart examples above, the most desirable quadrant is in the upper left (higher profit and lower cost) and the least desirable quadrant is the lower right (high cost and lower profits). Bubbles with larger areas have higher probabilities of success.
As you can see, studying a project portfolio in several bubble charts using different dimensions of business relationships can be a great help in making insightful business investment decisions.
Data Proportional to Radius or Area?Bubble charts can be misleading if care isn’t taken to understand the relationship between bubble size and the data the size represents. If the data is proportional to the bubble radius, the data will be skewed because bubble area grows exponentially as the square of the radius (Area=π*r^2). For a proportionally accurate representation of the data, bubble data should be represented directly by the area using the circumference of the circle, which grows the bubble area linearly in relationship to the diameter (circumference=π*diameter).
Bubble Chart Example 3: The same as Bubble Chart Example 1, but the bubble size has been changed to be proportional to the radius. Notice how the bubble sizes compare to Example 1.
This isn’t to say that one should never use the radius to represent the data as it can be useful if one wants to exaggerate the differences between the data objects or projects. For example, if the bubble data is similar in magnitude and you want more easily contrast the bubbles. However, it should be specified on the chart as to what representation is being made. In Bubble Chart Pro, one can choose between “Bubbles proportional to area” or “Bubbles proportional to radius,” and the selected relationship is stated on the resulting bubble chart.
Differentiating Bubbles in Bubble ChartsBubbles are usually differentiated by color, pattern, number labels, or a combination of these. Colors are usually adequate for small numbers of bubbles, but subtle differences in colors become difficult to distinguish in larger number of projects. Therefore, numbers corresponding to a chart legend becomes a more useful method of distinguishing bubbles.
Bubble Chart Example 4: The bubbles can now be distinguished by number labels in the center and the projects with probability of success values of less than 50% have a diagonal line pattern to distinguish them.
Crowded or Congested Bubble ChartsFinally, congestion and overlapping bubbles can be a problem with charts that have large numbers of bubbles or tight clusters of bubbles. This is a significant barrier to using bubble charts for analyzing such data, however, using features like the “zoom-boxes” in Bubble Chart Pro to instantly expand congested areas makes analyses of these types of charts much easier.
Bubble Chart Example 5: This shows how a Bubble Chart Pro “Zoom-Box” can be used to expand congested chart areas.