Objective : Allows you to relate 3 indicators to illustrate correlations. Perfect for comparing complex sets.
Application : Bubble graphs are generally used to compare and show the relationships between labels and categories, through the use of positioning and proportions. The bubble chart can be used to analyze patterns / correlations.
Too many bubbles can make the graph difficult to read. This graph therefore has a limited data volume capacity. This can be overcome by interactivity : clicking on bubbles can reveal hidden information, to go into more details.
For more information chart types.
Here an example of ToucanToco's Bubble charts :