### An example of a triple scatter plot or bubble chart

Reproduced from Cleveland WS, Mcgill R. Graphical Perception - Theory, Experimentation, and Application to the Development of Graphical Methods. Journal of the American Statistical Association 1984;79(387):531-54. The triple scatter plot or the ‘bubble chart’ has existed for a number of years, but is popularised through many colourful examples on the Gap Minder website (http://www.gapminder.org). Despite their attractiveness, they share the limitations that come with area charts; in particular humans are limited in the judgement of areas. In some cases, the data correspond neither to the diameter nor to the area of the ‘bubbles’, making the graph much harder to interpret. The sensible use of circles instead of other symbols is commendable because any partial overlap of circles results in a different shape, making individual circles distinguishable.