Today Culture of Insight tweeted the picture above of a slide from a presentation they were attending and made the point that the presenter was engaging in the rather dubious practice of y-axis shortening.
This is where the visualization of comparative changes is exaggerated by not starting the y-axis at zero.
This is something which is considered to be very bad visualization practice*.
To see why, play around with the interactive version below. Either by clicking on the “good”, “bad” and “as presented” buttons or by sliding the slider to see the different in overall impression when the Y-axis is not started at zero.
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*There are times when not starting your y-axis from zero can make sense (e.g. weather temperature charts) but in pretty much all business contexts the rule of starting from zero should be observed.