Good Time Charts, Bad Time Charts

April 21, 2015
Posted by Jay Livingston

How do you graph data to show changes over time? You might make “years” your x-axis and plot each year’s number. But you’re not the Washington Post. If you’re the Post Wonkblog (here), first you proclaim:

Here is that single chart.

(Click on a chart for a slightly larger view.)

The data points are years, but the seem to be in no logical order, and they overlap so much that you can’t tell which year is where.  Even for a point we can easily identify, 1987, it’s not clear what we are supposed to get.  In that year, the average income of the lower 90% of earners was about $33,000, and the average for the top 1% was about $500,000. But how was that different from 1980 or 1990. Go ahead, find those years. I’ll wait.

Here’s the same data,* different graph.

The point is clearer: beginning in 1980 or thereabouts, the 1% started pulling away from the lower 90%. 

A graph showing the percentage change shows the historical trends still more clearly.

From the mid-40s to 1980, incomes for the lower 90% were growing more rapidly than were incomes for the 1%. This period is what some now call “the great compression,” when income inequality decreased. Since 1980, income growth for the 90% has leveled off while incomes for the 1% have risen dramatically.

(The Post acknowledges that it got its material from Quoctrong Bui at NPR. But the NPR page (here) has two graphs, and the one that is similar to the one in the Post has an time-series animation that shows the year to year changes.)
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* The data set, available here , comes from the Paris School of Economics. Presumably, it contains the data that Thomas Piketty has been working with.