Dual-Axis Chart

A dual-axis chart overlays two different measures on one plot using two separate vertical scales — for example revenue (in dollars) on the left axis and complaints (in counts) on the right. It is tempting because it seems to show two stories at once in a compact space.

It is one of the most criticized chart types because the two scales are chosen by the maker, and sliding them up or down can make any two lines appear to track or diverge however the author wants — an illusion of correlation conjured from arbitrary scaling. In most cases two stacked small charts sharing a common horizontal axis tell the honest version of the same comparison.