A bubble chart is a scatterplot that shows relationships between three or four variables. The position of the bubble shows the relationship between the x and y variables.
• The bubble size is based upon a numerical variable, such as population, or sales.
• The bubble color is best reserved for categorical data, such as region.
• Bubble charts are best when the bubble sizes vary significantly.
Sosulski, Kristen. Data Visualization Made Simple (p. 51). Taylor and Francis. Kindle Edition.