A double line graph is a line graph with two lines. It has a row of numbers on two sides, showing the intervals, i.e. time, population, concentration of a substance. This is so it can be used to show the change and the rate of change, and the difference between the rates of change of two or more dynamic phenomenon.
Rarely one will see a row of numbers on the third side. This means there's a the third line on the graph which represents a phenomenon that cannot be measured with the same units. The phenomena are related, but they are really different topics. Take, for instance, a graph which shows the GDP (Gross Domestic Product) of the U.S., and the second line showing the GDP of China. A third line could represent the population of China. Because GDP is measured in different units than population, you need to use the third side of the graph. This would measure population. This way one can compare the population to GDP using the same intervals of time.