What Exactly is Happening in this 3D Chart?

When giving advice about chart styles, one of my friends likes to say “save 3D for the movies.” She’s right – research shows that 3D charts actually slow reader comprehension. I’ve long advocated for the use of ChartTamer as a friendly way to restrict data visualization to that which will actually support reader cognition. In a recent webinar I heard Cole Nussbaumer at Storytelling with Data give another compelling reason why we shouldn’t graph in 3D – Excel doesn’t use the front line or the back line of the 3D bar when aligning with the y axis – it graphs the midpoint of the bar. What??? Yes, she said Excel uses the middle of the 3D rendering as the determiner of the data point. I didn’t believe it. How could anything so antithetical to cognition actually survive in this sink-or-swim society? So I tried it myself:

Here is a 3D column chart of some fake data about customer satisfaction with prices in various departments of a natural health food store, before and after their physical move to a new location:

Now I know I’m breaking some serious graphing rules here, but stick with me. I super enlarged the y axis and honed in on one range of it so that we could really see where the gridlines line up with the tops of the columns.

See the blue bar in Produce? The actual data point for that blue bar is 85. Neither the front line, the back line, nor the midpoint of the top of the column are at 85. See the blue bar for Beauty? The actual data point there is 83. Again, none of the possible points of measurement on the 3D column are accurately expressing 83. Maybe more like 82.75, but definitely clearly absolutely not reaching the gridline marking 83.

I selected the exact same raw data table and created a similar column chart, just in 2D:

This time the blue bar in Produce is exactly at 85 and the blue bar in Beauty accurately represents the data point of 83.

I wasn’t able to reproduce Cole’s assertion that Excel uses the midpoint of the 3D column. But it is pretty clear that the audience misinterpretation often cited in research that results from the use of 3D charting is due to more than the complexity of analyzing in the third dimension – it is also because the columns simply aren’t accurately visualized.


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