The Café Wall Illusion makes you to see perfectly parallel lines as being at an angle. It’s a curiosity and a cool perceptual illusion – except when it shows up in a bar chart, as it did in this example.
The classic café wall illusion was first spotted on a tiled wall, where the misaligned black and white tiles were making the lines look all wonky (image from Wikipedia).
FiveThirtyEight journalist Allison McCann recently tweeted this image, which also shows the illusion. This is the first time I’ve ever seen it in a bar chart.
do u think the y axis line is straight y/n pic.twitter.com/N7lbyH28Nu
— Allison McCann (@atmccann) May 25, 2016
It looks like that zero line is angled up slightly. I wonder if this would cause people to be slightly biased in their reading of the data.