A bubble chart is used to represent two or three sets of data corresponding to a dimension. Two metrics take values in the two axes while the bubble radius represents the third metric.
When to use
Use bubble chart whenever you want to represent two or three metrics corresponding to a dimension.
Bubble charts are great at showing relationships among your data and conveying a large amount of numeric information quickly.
Consider the chart above. The dimensional value chosen is "Days of the month." The x-axis is a representation for Total Cost, Y axis for Avg CPC and the bubble thickness for Subscriptions (more the subscriptions of that dimensional value, more is the bubble radius). Therefore each bubble represents metrics (three here) for each dimensional value.
You can also use a bubble chart to represent two metrics for each dimensional value. In that case, the bubble radius would be same for all.
When two metrics are chosen the graph looks like the following (here Total cost and Avg CPC are the two metrics chosen)