The Why of the Y

When was the last time you pondered the Y-axis? Wait, you might be thinking, what’s the Y-axis again? In a typical chart (think bar chart, line graph), it’s the vertical axis. For example, in a bar chart, the Y-axis indicates the length of the bars and thus how much of something is in a category or at a certain point in time. The categories or time periods are indicated along the horizontal or X-axis. You might recall from your middle school days that the X and Y axes are part of the Cartesian coordinate system that René Descartes (pictured here) invented in the 17th century.

What's to ponder about a Y-axes? Well, at least two questions:

1. What should be the lowest and highest number on the axis?

2. What should be the interval between numbers on the axis?

The lowest point is called the “origin.” It’s where the Y and X axes intersect. Much has been written about the importance of starting the Y-axis at zero because, when you don’t, you can make a small difference look like a big one (see the two bar charts below for a case in point.) However, when all the numbers you are charting are not anywhere near zero, then starting at zero can make differences hard to detect (see the two line graphs below for examples.) And, if your high points are too high, your data will be crammed into the upper part of your chart, leaving a lot of useless empty space below.

Think of the low point and the high point as reference points for your data. Do you want to show progress compared to historic low or high points? Do you want to show progress in relation to goals? The answer to such questions will help you decide where to start and end your Y-axis.

As for what falls in between these two points, you should consider the range of your data points and how much accuracy and ease your viewer will want. If the data ranges from 2 to 12 with slight differences between points, then you might want intervals of .01. However, if the data ranges from 6 to 10,000, then intervals of 10, 100, or even 1,000 might be sufficient to give viewers a general, easy-to-interpret sense of the data.

If you’d like to ponder the Y-axis a bit further, check out this great video from Vox.

See other data tips in this series for more information on how to effectively visualize and make good use of your organization's data.

Frans Hals - Portret van René Descartes, André Hatala [e.a.] (1997) De eeuw van Rembrandt, Bruxelles: Crédit communal de Belgique, ISBN 2-908388-32-4.