Optsee® Bubble Charts

For background on 3-dimensional (3D) bubble charts, see "Optsee® 3-D Bubble Charts" by clicking here.

Optsee® 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 “Probability of Success (%)” (Y-Axis), “Cost” (X-Axis), and “Profit” (Bubble Size).

In this example, you can quickly see the "best" projects are in the upper left quadrant where the bubbles represent the the lower cost and higher probability of success projects. Within that quadrant, the projects with the larger bubble area represent the projects with the higher profit values.

Optsee® Bubble Charts in Project Portfolio Management

Optsee® bubble charts are often used in Project Portfolio Management 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, you can quickly see the "best" projects are in the upper left quadrant where the bubbles represent the the lower cost and higher probability of success projects. Within that quadrant, the projects with the larger bubble area represent the projects with the higher profit values.

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.

If you are tracking and modeling project attribute uncertainty, Optsee® bubble charts can also display that uncertainty in the form of bars drawn on top of the project bubbles. In Figure 2, the horizontal bars represent the maximum and minimum range of the cost associated with each project and the vertical bars represent the maximum and minimum range of the SMART ranking score calculated for each project.

Bubble Chart Example 2: The same as Bubble Chart Example 1, but displaying the bars representing the maximum and minimum ranges.

bubble chart with uncertainty bars

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). In Optsee®, you can choose to diplay the bubble size using either area or radius.

Bubble Chart Example 3: The same as Bubble Chart Example 2, but the bubble size has been changed to be proportional to the radius. Notice how the bubble sizes compare to Example 2.

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 Optsee®, 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 Charts

Bubbles are usually differentiated by color, pattern, number or name 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, names or numbers corresponding to a chart legend becomes a more useful method of distinguishing bubbles. In Optsee® you can display names, numbers, or modify the color or color-pattern combination for projects.

Bubble Chart Example 4: The bubbles can be distinguished by project names overlaying the bubbles. This allows for a bigger chart area because the legend can be hidden.

Hiding the bubble chart legend and using labels is another way to create more room in a bubble chart and easily identify the bubbles.

Crowded or Congested Bubble Charts

Finally, 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 Optsee® to instantly expand congested areas makes analyses of these types of charts much easier.

Bubble Chart Example 5: This shows how the Optsee® “Zoom-Box” feature can be used to expand congested chart areas.

Customized Bubble Charts

Optsee® bubble charts look great and they're easy to make: All the charts in Example 6 were created in less than a minute with just a few mouse clicks. And virtually all aspects of Optsee® bubble charts are fully customizable so you can create the look you want. You can easily set the colors and patterns for individual projects or groups of projects to distinguish them from other projects. You can also add your own background graphics to create dramatic looks with photographs, corporate logos, and textures.

Bubble Chart Example 6: This shows a few examples of different customized bubble charts you can create in Optsee®.

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