Definition: off the site
Graphics can provide insight when you are comparing two sets of data. For example, suppose we wanted to know if more people are finishing college today than in 1990. We could draw a side-by-side graph to compare the two data sets. Date sets should be compared by using relative frequencies, because different sample or population sizes make comparisons using frequencies difficult or misleading. However, when making comparisons, relative frequencies alone are not sufficient.

Definition: in my own words
making two graphs to compare the differences between each other