Tuesday, June 28, 2016

Defying conventions in visualization: Should time always be on the horizontal axis?

The main picture on the first page of today's The New York Times is a very nice time-series line graph by Alicia Parlapiano. Notice that time is on the Y-axis. You've probably heard or read that time in statistical graphics like this should always be on the horizontal axis because it feels natural, and that if you do otherwise, readers will be confused.

Could this be a cultural convention? In Western societies the passage of time maps onto a virtual, generally horizontal linear scale: before-after translates into “behind me” and ”ahead of me”, and this scale has a left-to-right orientation. Other cultures and languages (see 1, 2, 3) use both horizontal and vertical metaphors to think and talk about time. It'd be great to do some experiments and see if this has an effect on how people read charts.

As for the objection that readers —mostly Western ones here, I guess— will be confused, well, people aren't stupid. They may be puzzled in the first 5 seconds, but only until they take a quick look at the axis labels. When reading graphics, attention overrides preconceived notions.

Hunches aside, I usually recommend to follow conventions unless there's a good reason not to. This is one of those cases. There's a true cultural metaphor at play in this chart: the more liberal-more conservative spectrum, which translates into a left-right scale. If we put time on the horizontal axis, and the left-right scale on the vertical one, the latter would map as higher-lower (update: this is how it shows in the online version, h/t Nathaniel Lash.)

As a final note, here's a prediction: as a majority of readers are accessing their news through smartphones —latest figure I heard from a major news organization in Miami is 80%,— 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?

Update: On Twitter, Álvaro Valiño has shared this ISOTYPE chart.


  1. Since the NYT presented the labels horizontally this presentation seems just fine to me.

  2. Even in this example, I prefer time on the x-axis. There's a number of people that have visualized this data and I think it works really well in the traditional format of time on the x-axis. I really like the NY Times version of it.


    What's really interesting about this version though, is that it's right on point to your comment (and prediction) about smart phones. When I open this link on my computer, tablet or rotated phone then time is on the x-axis, but when I look at it on my phone in normal position, then time is converted to the y-axis so that it fits better on the screen.

  3. It isn't just a Western thing. I've asked Asian and Israeli students how timeline charts are presented in their countries and they all respond that time goes from left to right. Indeed, a Google search (using the Google home page associated with China, Japan, and Israel) supports this.

    When I teach my classes I don't pronounce "time must go from left to right." Rather, I have students swap the axis with a bar chart and then swap the axis with a line chart.

    I then ask why they have so much trouble reading the line when it is vertical. Many respond that they grew up looking at it a particular way and that way is hard to shake.

    As for mobile devices helping the adoption of vertical timelines. Yes, it is possible to shake tendencies that we grew up with. Just look at how well everyone in the USA did with the metric system.

    PS To my surprise I didn't have a problem with the NYT graph. I guess "left / right" representing liberal and conservative trumped "left / right" representing old and new.

    1. Yes, non-Western cultures also use left-right metaphors for time. My point was that some languages, like Mandarin, seem use up-down metaphors widely (according to the papers linked above), so I was wondering if that has an effect on how easily they read charts in which time is on the Y-axis, in comparison to Western readers

  4. Actually, I think the oddest thing about all this is that Alberto has a hard-copy of the New York Times.

    1. I am much more old fashioned than you are, Steve. Happiness for me is the print New York Times and a cup of black coffee early in the morning, when everybody else is sleeping.

      (After I read the print paper I check news online, of course)

  5. Funny, when I first glanced at this chart, I thought it was going to read from bottom to top.