The Bubble Chart

What is a Bubble Chart?

A bubble chart is a data visualization that is similar to a scatter plot, but supports another dimension of data. Like the scatter plot, one variable has values which lie on the x-axis, and the other on the y-axis. In addition to this, bubble charts add another dimension of data, which appears as the size of the data point. Unlike scatter plots, where each data point is the same size, bubble charts allow the data points to differ in size. The larger the data point, the larger the value of the third variable. Also, a bubble chart has the capability of accommodating even 4 or 5 dimensions of data corresponding to the bubble’s label and category.

Bubble charts allow you to compress a large amount of data into a single data visualization. With the ability to view up to 5 dimensions of data, a large table can be presented as a single visualization, as long as it is set up properly.

When to Use a Bubble Chart

Bubble charts, like other visualization tools, are a form of storytelling. They represent the relationship between data using the position and proportion of the plotted bubbles. The bubble chart is particularly helpful to understand data sets with multiple variables, visualize patterns, and identify trends using data analysis.

 

Chart made using Chartio

In the example above, the bubble chart depicts UFO sightings. On the x-axis, the UFOs are separated into the shape of UFO spotted. On the y-axis is the number of distinct sightings of each UFO shape. Finally, the size of the bubble indicates the total time that each type of UFO was observed. The larger the bubble, the longer the duration of time each UFO was spotted. In this example, the bubble points are separated enough that we can fully distinguish each bubble from one another.

When NOT to Use a Bubble Chart

Because the size of the bubble takes up more space than a typical scatter plot, it is easy for a bubble chart to become overcrowded with a large number of data points. When a bubble chart becomes overcrowded, it makes it very difficult to gain any information from the visualization for analysis. In the example below, the bubble chart depicts the UFO sightings by year, with the year on the x-axis and the number of sightings on the y-axis. Additionally, the length in time that each UFO was sighted is shown by the size of the bubble.

Chart made using Chartio

There are two main issues that make this bubble chart an ineffective data visualization: the size and overlapping of the bubbles, with the overlapping being the major problem. There are simply too many data points on this chart causing major overlapping. This in turn makes the size and position of many of the bubbles non distinguishable from one another, ultimately making this a hard-to-interpret visualization. The size of the bubbles also contributes to the overlapping, but is harder to control due to the size representing a dimension of the data. Overall, this visualization is too crowded and the data could be better represented by a different chart type.

Comparison of Like Chart Types

The bubble chart is a useful type of visualization which can be used to show the relationship between more than two quantitative variables. The scatter plot is another data visualization which compares two quantitative variables. The table below gives the use case and pros and cons of the scatter plot and bubble chart:

Scatter PlotBubble Chart
Use
  • Compare two quantitative variables
  • Compare three quantitative variables
Pros
  • Show larger quantities of data
  • Easy to see clustering and correlation
  • Add another dimension (size of bubble) to the scatter plot for more comparison
Cons
  • Useless if no clustering or trend exists
  • Bubble size can make chart crowded

 

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