Keep An Eye On Your Y-Axis

A line graph with a y-axis that doesn't start at zero presents a misleading picture of the data.

Graphs are a useful way to communicate data succinctly, and to emphasize important or surprising findings. Like the graph above, where I plotted the percentage of total land area that’s made up of forests. A quick glance shows that the situation is pretty dire, right? I mean, look at that steep drop from 1990 to 2016!

Well, not so fast. Take a look at the Y-axis there (the one that runs vertically). It doesn’t start at zero. That makes a big difference, because it’s basically exaggerating what’s changing over time.

A Slight Change Makes A Big Difference

Take a look at the graph below — same data, same format, but now with the Y-axis starting at zero. Do you see the difference? The decrease since 1990 is still apparent, but it’s very slight.

A line graph with a y-axis that does start at zero presents the data in a more objective, honest light..

Graph With Your Critical Eye

It’s so easy today with MS Excel and other software that automatically create graphs and charts to just let the computer take care of things. They’re built to save us time, right? But when you do that, you’re relying on the defaults — basically, what the software designers thought would work in every situation.

Not starting your Y-axis at zero introduces bias into your graphs. It exaggerates the scope of the difference between two data points. Now, maybe it’s important to show that, to really zero-in (no pun intended!) on a small segment of your chart. But be aware that doing so can very easily lead your reader to wrong conclusions about the entire graph.

Quick Fix

So take a look at your graphs once you’ve created them, and make sure you’re all set with where your Y-axis begins. If you need to manually set it to start at zero:

  • right-click on the axis;
  • select Format Axis; and,
  • under Axis Options set the minimum to “0.”

You’re all set!

Sign up for my newsletter and get more helpful tips.

No spam, just interesting data tips and suggestions landing in your Inbox once a week or so. You can sign up here.

2 thoughts on “Keep An Eye On Your Y-Axis

  1. Julie, your observation made a great deal of sense. I will pay more attention to the Y axis when I look at graphs!

Leave a Reply