What is the difference between the two bar plots below?
I am sitting on a conference and these type of plots are relatively frequent in the presentations. Complete with a log-scale.
The answer is, of course, that there is no difference between these two — the data is exactly the same, the only thing different is the vertical scale. These two plots explain why you should never, ever use a bar plot to represent log-scaled data: the position of the y axis is completely arbitrary, yet it influences greatly our perception of which plot shows a larger difference.
(See also “Kick the bar chart habit”)