Misleading Visualizations: Cropping Axis on Bar Graphs

One of the most common problems in graphs is the use of a cropped axis in a bar graph. This can lead to a visually overstated difference between two numbers. In the graph on the left, which was automatically was created by Excel, Campaign B looks greatly more successful than the other campaigns in terms of average gift. Though campaign B was more successful than the other campaigns, the average gift for Campaign B is only 10% higher than Campaign A. However, in a graph with an axis that starts above 0 like the graph on the left, the bar for Campaign B is more than twice the size of the bar for Campaign A, leading to a misleading visual representation of the data. The graph on the right shows an accurate visualization of the data, with the axis starting at 0.

But what if you want to emphasize the differences in the data, even if they are a small percentage of the data (i.e. a difference of $1000 in bars that are in the millions)? In this case, you have a few options. You can use a table containing the numbers and/or differences from each other or an average. Not everything needs to be visually represented, particularly when their are only a few categories. Another option is to use a line graph with a cropped axis. Because a line does not have a solid area like a bar, the differences in size do not confuse the audience in a line graph as as they would in a bar graph. One more way to represent this data is to visualize it in terms of difference from an overall average or other reference point. You can determine method which works best, just avoid cropping axis for any graph with solid area.