We do not perceive area as accurately as we perceive length, particularly when comparing areas of different sized circles or irregular shapes. For example, we can fairly accurately judge whether a bar is twice as tall as another, but cannot judge whether a circle is twice as large as another. Comparative circle size (‘bubble’ charts) may be acceptable for conveying a general impression – that is, ‘this one is bigger than that one’ – but not for accuracy. Consider using bars instead if accurate perception of the relative sizes is critical.

Using pictures to replace bars on graphs (ie an illustrated graph) should also be done with care – if a picture of a tree is scaled up proportionally, are readers expected to consider only the change in height or the change in area as well? Also, it may be unclear where the actual data value is in the picture – is it the tip of the highest blade on a wind turbine or is it the top of the main tower?