A twitter user alerted me to this chart put out by the Biden adminstration trumpeting a reduction in the budget deficit from 2020 to 2021:
This column chart embodies a form that is popular in many presentations, including in scientific journals. It's deficient in so many ways it's a marvel how it continues to live.
There are just two numbers: -3132 and -2772. Their difference is $360 billion, which is less than 10 percent of the earlier number. It's not clear what any data graphic can add.
Indeed, the chart does not do much. It obscures the actual data. What is the budget deficit in 2020? Readers must look at the axis labels, and judge that it's about a quarter of the way between 3000 and 3500. Five hundred quartered is 125. So it's roughly $3.125 trillion. Similarly, the 2021 number is slightly above the halfway point between 2,500 and 3,000.
These numbers are upside down. Taller columns are bad! Shortening the columns is good. It's all counter intuitive.
Column charts encode data in the heights of the columns. The designer apparently wants readers to believe the deficit has been cut by about a third.
As usual, this deception is achieved by cutting the column chart off at its knees. Removing equal sections of each column destroys the propotionality of the heights.
Why hold back? Here's a version of the chart showing the deficit was cut by half:
The relative percent reduction depends on where the baseline is placed. The only defensible baseline is the zero baseline. That's the only setting under which the relative percent reduction is accurately represented visually.
This same problem presents itself subtly in Covid-19 vaccine studies. I explain in this post, which I rate as one of my best Covid-19 posts. Check it out!