An extension of a scatterplot, a bubble chart is commonly used to visualize relationships between three or more numeric variables. Each bubble in a chart represents a single data point. The values for each bubble are encoded by 1) its horizontal position on the x-axis, 2) its vertical position on the y-axis, and 3) the size of the bubble. Sometimes, the color of the bubble or its movement in animation can represent more dimensions.
I say surprisingly tricky because it’s easy to overwhelm the user when trying to view bubble charts. I think the best scenarios are cases in which you have relatively few data points and the size element is mandatory.
Hans Rosling (RIP) did an outstanding job of displaying this kind of chart with the Gapminder dataset.