The “On…” series is a collection of short blog posts relating to data visualization, economics, presentation skills, or data communication. In each, I discuss an issue, concept, or idea that I have not fully developed, a work in progress, or just some thoughts about a topic or issue I’d like to share.
Maarten Lambrechts made a good point the other day on Twitter:
A y-axis on the left is almost always the default, while the most recent and usually most relevant data are on the right. Then shouldn’t a y-axis on the right be the default (when there are no data labels), to improve legibility? pic.twitter.com/Gt5DmvguyH
— Maarten Lambrechts (@maartenzam) September 17, 2020
There doesn’t seem to be any reason why we can’t put our vertical axis labels on the right side of the graph. As Maarten showed in his tweet, placing the labels on the right side of that particular graph makes the quantities clear.
Where did the left-aligned vertical axis come from? William Playfair placed the vertical axis in different positions in his early work–on the right side in his famous Exports/Imports graph and on both sides in the price of wheat bar/line chart.
Two ideas on where the current practice originated:
First, we read English left-to-right, so putting the vertical axis labels on the left side is consistent with that direction. Over time, maybe this just became the default? Furthermore, in the left side location, the labels are positioned closer to the title/subtitle area and thus all of the labeling/scaffolding text is in a similar position. Research from Michelle Borkin and colleagues has shown that people are very likely to read text in and around graphs, so this placement might help tap into that propensity to read text.
Second, as Cole Nussbaumer Knaflic noted on Twitter, with graphs that show positive values, we are plotting our data in the upper-right section of the Cartesian Coordinate System, which has those natural left-hand side labels.
In any case, placing the vertical labels on the right side of the graph seems like a good strategy. In cases where placing the labels on the right side seems useful, as in the graph Maarten showed, I’d lean towards including them on both sides because it still seems natural to see those labels on the left side of the graph. It will be interesting to see if more people adopt this approach.