Elevator shoes for column charts

Continuing my review of some charts spammed to me, I wasn’t expecting to find any interest in the following:

It’s a column chart showing the number of years of data available for different asset classes. The color has little value other than to subtly draw the reader’s attention to the bar called “Art,” which is the focus of the marketing copy.

Do the column heights encode the data?

The answer is no.

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Let’s take a little journey. First I notice there is a grid behind the column chart, hanging above the baseline.

I marked out two columns with values 50 and 25, so the second column should be exactly half the height of the first. Each column consists of two parts, the first overlapping the grid while the second connecting the bottom of the grid to the baseline. The second part is a constant for every column; I label this distance Y.  

Against the grid, the column “50” spans 9 cells while the column “25” spans 4 cells. I label the grid height X. Now, if the first column is twice the height of the second, the equation: 9X + Y = 2*(4X+Y) should hold.

The only solution to this equation is X = Y. In other words, the distance between the bottom of the grid to the baseline must be exactly the height of one grid cell if the column heights were to faithfully represent the data. Well – it’s obvious that the former is larger than the latter.

In the revision, I have chopped off the excess height by moving the baseline upwards.

That’s the mechanics. Now, figuring out the motivation is another matter.