Random Fact: What the eye really sees

"We make the bars in bar graphs and histograms equal in width because the eye responds to their area. That's roughly true. Careful study by statistician William Cleveland shows that our eyes "see" the size of a bar in proportion to the 0.7 power of its area. Suppose, for example, that one figure in a pictogram is both twice as high and twice as wide as another. The area of the bigger figure is 4 times that of the smaller. But we see the size of the bigger figure as 2.6 times the smaller, because 2.6 is 4 to the 0.7 power."

Moore, David S. Statistics: concepts and controversies. 5th ed. New York: W. H. freeman and Company, 2001.