Visualizing change over time: case study via Arstechnica
Book Review: Visualizing with Text by Richard Brath

Locating the political center

I mentioned the September special edition of Bloomberg Businessweek on the election in this prior post. Today, I'm featuring another data visualization from the magazine.



Here are the rightmost two charts.

Bloomberg_politicalcenter_rightside Time runs from top to bottom, spanning four decades.

Each chart covers a political issue. These two charts concern abortion and marijuana.

The marijuana question (far right) has only two answers, legalize or don't legalize. The underlying data measure the proportions of people agreeing to each point of view. Roughly three-quarters of the population disagreed with legalization in 1980 while two-thirds agree with it in 2020.

Notice that there are no horizontal axis labels. This is a great editorial decision. Only coarse trends are of interest here. It's not hard to figure out the relative proportions. Adding labels would just clutter up the display.

By contrast, the abortion question has three answer choices. The middle option is "Sometimes," which is represented by a white color, with a dot pattern. This is an issue on which public opinion in aggregate has barely shifted over time.

The charts are organized in a small-multiples format. It's likely that readers are consuming each chart individually.


What about the dashed line that splits each chart in half? Why is it there?

The vertical line assists our perception of the proportions. Think of it as a single gridline.

In fact, this line is underplayed. The headline of the article is "tracking the political center." Where is the center?

Until now, we've paid attention to the boundaries between the differently colored areas. But those boundaries do not locate the political center!

The vertical dashed line is the political center; it represents the view of the median American. In 1980, the line sat inside the gray section, meaning the median American opposed legalizing marijuana. But the prevalent view was losing support over time and by 2010, there wer more Americans wanting to legalize marijuana than not. This is when the vertical line crossed into the green zone.

The following charts draw attention to the middle line, instead of the color boundaries:

Junkcharts_redo_bloombergpoliticalcenterrightsideOn these charts, as you glance down the middle line, you can see that for abortion, the political center has never exited the middle category while for marijuana, the median American didn't want to legalize it until an inflection point was reached around 2010.

I highlight these inflection points with yellow dots.


The effect on readers is entirely changed. The original charts draw attention to the areas first while the new charts pull your eyes to the vertical line.



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