Cropped axes — a flag for misrepresentation

Originally published at c306.net on March 21, 2016.

Original, misleading chart with cropped axis

Saw this chart — on prices in a car segment in the US — in this article in The Economist. I just couldn’t get over how grossly it misrepresented the data, so here’s the crib.

Illusion

On a quick look, the chart above seems to indicate that the US government discounts electric car prices by anywhere from 37.5% (BMW i3) to 60% (Tesla Model 3) of the total price — the ratio of light blue bar segment to total.

The reason for this apparent fallacy is the cropped axis on which the values are plotted — instead of starting from zero, it starts from about $22,500 and goes up to $45,000. Adding to the crime of cropping is the location of the axis labels — in the already text heavy section on top.

Result: most viewers would casually interpret that tax credits form a huge rebate on electric cars.

Reality

Same chart with uncropped axis, giving a different picture

The interpretation changes quickly when the axis is expanded to start from zero.

The tax credit is now clear as just a small share of the real price — a standard $7,500, whatever the car price — rather than a hefty 60% for a Tesla Model 3.

What’s more, it now highlights how competitively priced the Tesla Model 3 and Chevy Bolt are, even if the price subsidy is removed — only BMW 3 series beats them. And that’s before the running & servicing cost savings are accounted for.

Don’t crop the axis!!

Based on my experience of reading, and creating, countless charts — one of my key learnings is simply:

If the axis are cropped, the chart creator is trying to send a false reading — so dig in deeper.

In the case of this chart, the cropping is compounded by hiding the labels in the text heavy section of the graphic.

It’s either a super lazy job by the chart making (and the editor). Or a case of making the chart fit the story / bias.

Please, don’t do it.

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