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.