I keep seeing accusations that graphs where the y axis doesn’t start at 0 are misleading. Sometimes these accusations are correct. Frequently, however, these accusations are unfounded.
It’s certainly true that the relevant axis for a bar chart should include 0. But that’s not necessarily the y axis and it doesn’t necessarily mean the axis should start there; If we have positive and negative axes then 0 should appear somewhere in the middle.
Unlike bar charts, line charts and many other kinds of chart don’t encode data using length. So these charts have different rules. Yes, line charts that don’t include 0 on the y axis can be used to mislead. But so can line charts that do explicitly include 0 on the y axis, as so well demonstrated by the chart here which emphasises the supposed importance of the utterly meaningless 0 Fahrenheit in an attempt to prove a point. Unfortunately for the author, the only point it really proves is that they should leave climate science to those who actually understand temperature scales (some of the responses are excellent, however).
And why is it always the y axis people focus on? People rarely claim a time series should go back to the very start of a data series (or the Big Bang). Somehow removing empty space is seen as more of a sin than removing space that actually contains data!
It’s not just me who gets annoyed by this; if it was I’d start to wonder if I was just plain wrong. So I thought I’d collect together a list of articles expressing the same point. If you’re still not convinced by the above, try reading the articles below. If you’ve got any more suggestions for the list or would still like to tell me that I’m wrong then feel free to leave a comment.
- Naomi Robbins – Must Zero Be Included on Scales of Graphs? Another Look at Fox News’ Graph and Huff’s Gee-Whiz Graph
- Edward Tufte – Baselines
- Vox – Shut up about the y-axis. It shouldn’t always start at zero.
- Jorge Camoes – Bars and lines: méfiez-vous des morceaux choisis
- Xan Gregg – John Tukey on the rule of zero-origin scales
- Quartz – It’s OK not to start your y-axis at zero
- Justin Fox – The Rise of the Y-Axis-Zero Fundamentalists
- Me – Line Charts: Where to Start?