If you are familiar with single line graphs, you know that you can plot some variable against time or another variable. For example you might want to see the number of students attending your favorite university for each of the last 10 years.
Now if you can plot one line on a graph, you can plot more than one or many. You could plot on the same graph as above, the number of female student and the number of male students for each of the last 10 years.
Your result would not be a double line graph, but a triple line graph.
