While many graph types geared toward comparisons ask the viewer to subtract the difference between the heights of two bars or the space between two points on a line, a deviation bar graph simply graphs the difference. A deviation bar graph encodes the change between two points. The two points can be points in time, the difference between two groups, the change from pre to posttest, or any other dataset suitable for comparison.
Deviation bar graphs lose some detail. The exact percentages of the two points in comparison are not reported. Rather, the difference is reported. Thus a deviation bar graph emphasises the change and asks the viewer to focus on that aspect alone.