The photograph above is taken from Alberto Cairo's brilliant book on infographics, 'The Functional Art' - page 121.
What I absolutely love about this book is that Alberto takes a science-based approach, and explores in real detail how different visual representations of data can either either make it easy or difficult for users to understand and interpret data - depending on the form chosen.
In this case, he looks at how, despite their recent upsurge in popularity, the use of bubble charts do not help us make meaningful judgements from the data if the aim is comparison.
When we look at circles, we are not in fact very good at comparing areas - we rather tend to compare their relative heights. This means that we struggle to accurately estimate the relative difference in values when they are represented by areas. So if our aim is to compare amounts, then as boring as they may be for graphic designers whose overriding concerns are visual, in this instance, bar charts are in fact, far superior. They help us do what the graph was ostensibly developed for in the first place - to help users interpret and understand the data better.