Time on the y-axis

Normally, charts have time on the x-axis, moving from left to right. Earlier this year, Alberto Cairo wrote an article on charts that have time on the y-axis. This may make practical sense if you want to show developments over time on a political left-right scale. He also pointed to the use of mobile screens:

As a final note, here’s a prediction: as a majority of readers are accessing their news through smartphones […] which are usually held upright and navigated by scrolling vertically, vertical time-series charts with time on the Y-axis will become more common in the next few years. Will we witness a new visual convention being born?

Now Kaiser Fung discusses a few charts by the Washington Post (aptly described as troll hair charts) and the New York Times that also have time on the y-axis. They’ve made different choices regarding the direction of time: «The Post’s choice of top to bottom seems more natural to me than the Times’s reverse order but I am guessing some of you may have different inclinations.» Which suggests that the convention of showing time on the y-axis hasn’t crystallised yet.

Based on the connection with scrolling on mobile screens, the Washington Post’s top-to-bottom approach may well emerge as the standard approach.