Your Y-axis Might be Wrong…

Written by Kally Pan on May 04, 2016

Representing 2-dimensional data can be a challenge; visualizations like line and bar graphs suffer from a common problem: where do you set the y-axis?

Truncating the y-axis willy nilly will exaggerate differences in the data, leading to these abominations:

wrong about y axis blog 1

wrong about y axis blog 2

Just terrible.A common rule of thumb that people are using to avoid is to start every y axis at 0. However, as this great video from Vox shows, that’s not always the right thing to do either.

Why? As we have often preached: the most important thing to include in your chart is context, context that help you understand the data and how it fits into the narrative you are presenting. In some cases, 0 is a helpful baseline that your data rests on and in those cases, by all means include it in your axis.

But in other cases, you might need to start above or even below the 0 axis. A chart of body temperatures of a fever patient shouldn’t have a y-axis that starts at 0, and a chart of historic temperatures in Antarctica probably needs to start well below 0.

In any case, think first to what story the overall data is telling and then pick a visualization that best illustrates that point. There wasn’t anything inherently wrong with the visualizations found at the top of this article, the problem was that they were being used to tell a narrative that was grossly exaggerated.

In the first case, it might help to know that tax rates in the last 50 years have spanned a range that’s 5 times more than the entire y-axis in that chart. In the second case, it might help to know that major league knuckleball speeds can vary between 60-75 mph. Both are much more logical ways of depicting the data if you want to give it additional context.

Have you seen any examples of poorly placed y-axis? Or do you have any insights you’d like to share about this issue? Let us know, we’ll feature it in a future post if it leads to an interesting discussion. Cheers!