When is changing a graph axis OK?

One of the things I find deceiving when looking at graphs on slides is when the vertical axis on a column chart has no values. In many cases, this means that the scale does not start at zero and it means the presenter is trying to fool their audience into thinking that the graph tells a story that is better than reality. Check out most of the graphs you see in corporate quarterly analyst briefings and you'll see what I mean.

But recently I changed the scale on a column chart to make the point more effective. On a client slide they had two column charts - one to show additional revenue over the next four years and another to show the investment required to realize that revenue. Initially, they had both charts side by side with the scale of the revenue chart going from 0 to 120 and the investment chart scale 0 to 25. The problem was that visually, the height of the investment bars was almost as high as the revenue bars, making it look like the return would be very small. Most observers won't make the connection that the scales were different. So I changed the scale of the investment chart to be the same as the revenue chart. Now, the side by side charts show that the investment is quite small compared to the revenue that is gained.

When you are using more than one related chart to make a point, make sure that the visual comparison can tell the correct story without your audience having to read the scales of the charts.