Steven Sanderson creates a bubble chart:
Bubble charts are a great way to visualize data with three dimensions. The size of the bubbles represents a third variable, which can be used to show the importance of that variable or to identify relationships between the three variables.
To create a bubble chart in R using
ggplot2, you will need to use thegeom_point()function. This function will plot points on your chart, and you can use the size aesthetic to control the size of the points.
Click through for two examples, one which is a pretty good outcome for using a bubble chart, and one which exposes the key weakness of bubble charts.