The Same Series, Wide and Short or Narrow and Tall

Both panels plot the identical annual unemployment rate from 1960 to 2014 on identical vertical scales. The only difference is the shape of the box the line is drawn in. Spread wide and flat, the rise in 2009 looks like one bump among many. Squeezed narrow and tall, the same rise looks like a cliff.

Neither panel is dishonest. Every point is in the right place in both. That is what makes the trick worth knowing: a reader who takes an impression from the shape of a line, without checking the axes, can be led a long way by a choice the author was free to make for any reason at all. The slider takes the stretch from mild to extreme.