Be Careful with the Y-Axis…

Oct 6th, 2015 / Data Visualization, Presentations / , , , , ,

I was recently working with an organization to improve their graphs for an upcoming conference presentation.
Notice anything funny about the height of these bars?
The shortest bar, for 2012, represents 26,000 youth. But the tallest bar, for 2010, represents 27,000 youth. 26,000 and 27,000 are pretty similar numbers, yet they look reeaaaallllly different. The 26,000 seemed way too short compared to the 27,000.
This graph’s vertical y-axis doesn’t start at zero, so the differences between bar heights are exaggerated.
Before: The y-axis doesn't start at zero, yet isn't labeled
 
It’s okay to have a non-zero y-axis. But, it must be labeled.
In the after version, I added labels for 25,600, 26,000, 26,400, and so on.
We don’t want to mislead our viewers. We have to be clear that we’ve intentionally truncated that y-axis so that we could zoom in on that segment between 26,000 and 27,000.After: The y-axis doesn't start at zero, but now there's a label
 
Or, another option is to adjust your vertical y-axis so that it starts at zero. Then, you wouldn’t need to have any axis labels off to the left of your graph.
The tradeoff is that, now, the bars are all roughly the same height… which might be okay, depending on what you want to emphasize. Maybe you’re trying to show that a consistent number of youth were enrolled in the study sample each year. In that case, starting the y-axis at zero would help you out.
Another option is to start the y-axis at zero, which wouldn't require axis labels
 
Let’s look at those two “after” versions once more.
Option A, on the top right: If you’re going to start your y-axis at something other than zero, then you need to add axis labels.
Option B, on the bottom right: You can start your axis at zero and forego having any labels.
Two options for fixing a truncated y-axis
How have you handled truncated y-axes in your projects?
 

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