Regular readers will know that I have a bit of a Thing about bad graphs used in the media and on blogs. When people use stupid presentation tricks to exaggerate features of data to make their argument look stronger, it bugs me. But what really irks me is when people use stupid presentation tricks to trample their own arguments.
The blue line is debt, the reddish one income (adjusted for inflation, I believe). Both data sets are normalized so that the 1980 value is 1.00. You can’t tell that easily, because for no good reason, Konczal has chosen to bury his point by plotting them on separate vertical scales– the red line uses the numbers on the right side of the graph, while the blue line uses those on the left.
“I don’t know,” you say. “If they weren’t on different scales, you wouldn’t be able to see the variation in the wages line.”
Yes, I know. And that’s the whole point of the graph.
This is intended to show that consumer debt absolutely exploded relative to income in the last ten years. But the point it partially obscured by the fact that the lines are on two different scales. Visually, it looks like income started higher than debt, and grew by about 50% (from roughly 4 on the left-hand scale to roughly 6). Debt clearly grew faster, but it looks like the two passed each other at around 2002.
In fact, the red line uses the right-hand scale, where zero is somewhere way off the bottom of the plot. The actual growth in income was about 20%. There was maybe a brief period in the early 80′s when income exceeded debt on this scale, but the growth was nowhere near comparable. Here’s a really crude mock-up of what it would look if the two were plotted on the same scale:
The green line is roughly what the red line would be if it were on the same scale as the blue line. I haven’t attempted to reproduce all the little wiggles, because they don’t matter on this scale. From 1980-2007, income increased 20%, and debt increased 1000%.
And there’s no reason why they can’t be plotted on the same scale– they’re both dimensionless quantities, having been normalized to 1980 values. In fact, putting them on the same scale makes the important point clearer than having them on different scales.
Now, this is being a little unfair, as the normalization hides the relative sizes of the initial numbers. One would hope that income exceeded debt for much of this period, and if it did, then starting both at 1 would confuse the issue in a different way. A more accurate presentation might use absolute numbers adjusted for inflation, or represent both as a fraction of inflation-adjusted income, or something like that.
And, to be fair, both Drum and Konczal talk about these as if the really important thing is the tiny change in the average slope of the red line around 2000, where it goes from infinitesimally upward to essentially flat. You might miss that change on the current scale, and you’d definitely miss it on my preferred graph. Then again, the divergence between the two curves at that time would probably be even more dramatic if they were plotted in a sensible way.
The simple, general lesson to draw from this is: Think carefully before using a two-axis plot. Make sure you’re not trampling your own point with a lousy presentation format.