A stacked area chart is the extension of a basic area chart to display the evolution of the value of several groups on the same graphic. The values of each group are displayed on top of each other. It allows to check on the
same figure the evolution of both the total of a numeric variable, and the importance of each group. If only the relative importance of each group interests you, you should probably draw a percent stacked area chart. Note that
this chart becomes hard to read if too many groups are displayed and if the patterns are really different between groups. In this case, think about using faceting instead.