The Café Wall Illusion in a Bar Chart
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).
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.