Another day, another unnecessary dual-axis chart. Is it just me or is this often misleading graphic form —barely appropriate in very specific cases (1, 2)— becoming more popular?
After the terrible example from a few days ago, I've just seen another one in Robert B. Reich's otherwise interesting Saving Capitalism: For the Many, not for the Few. Reich has mastered the use of motion and static graphics to persuade. Just watch his famous documentary; here's a glimpse. But persuasion mustn't be our primary goal when designing a graphic. Accuracy and truthfulness are.
See the original and a quick redesign:
I'll concede that in this case the distortion isn't that bad but, if you can use the same correct scale for all your data, why not doing it? Why stretching and compressing the lines and give the impression of a perfect co-variation?
(Chiqui Esteban has suggested that an indexed chart would be better here. I agree, but I don't have access to the data.)
Quick update: I finished writing The Truthful Art last week. I'll be proofreading until the end of November, so the book can be released in early March 2016. Blurbs have begun coming in. Here are Jeff Jarvis's and Michael E. Mann's:
And here's a screenshot of a page that deals precisely with dual-scale charts. The quote by Gary Smith, “If you double the axes, you can double the mischief”, comes from hist most recent book, Standard Deviations.