Bubble chart: definition, examples, and best practices.

What is a bubble chart?

A bubble chart starts from the same layout as a scatter plot, plotting two variables on two axes, then adds a third variable by scaling the size of each point. A bigger bubble means a bigger value on that third measure, so three dimensions of a dataset become visible at once without needing a third axis.

The type is also called a bubble plot. Because size is judged by area rather than by a single dimension like height, it needs a bit more care than a standard scatter plot to stay readable and honest.

A worked example

Export diversity index against per capita exports for 24 economies, with point area proportional to GDP. Four archetypes separate out: concentrated resource exporters at the top left, diversified high earners at the top right, and low-income economies split by how diversified they already are. Six of 24 points shown.

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When to use a bubble chart

• Assessing market entry options by plotting GDP per capita against growth rate, sized by total market value.
• Comparing products by price against customer satisfaction, sized by unit sales volume.
• Evaluating an investment portfolio by risk against return, sized by position size.
• Presenting a competitive landscape by two performance metrics, sized by company revenue or headcount.

When not to use a bubble chart

• If you only have two variables to show, a scatter plot is simpler and avoids the added complexity of judging bubble size.
• If you have more than about fifteen points, overlapping bubbles get hard to compare. A marimekko chart can sometimes carry a similar multi-dimensional story with less overlap.

How to read a bubble chart

Read position first, exactly as you would on a scatter plot, to see how the two axis variables relate. Then read bubble size for the third variable, remembering that size should scale by area, not by diameter, so a bubble twice as wide represents roughly four times the value, not two.

Best practices

• Scale bubble size by area, not by radius or diameter, so relative size is not visually exaggerated.
• Cap the number of bubbles at around fifteen. Beyond that, overlapping bubbles become impossible to compare accurately.
• Add a size legend showing what a small, medium and large bubble represent in real units.
• Use transparency on the fill so overlapping bubbles remain distinguishable rather than blending into a solid mass.
• Label only the bubbles that matter to the story. Labelling every bubble on a crowded chart makes it unreadable.