Line graphs are most often used to show how certain values change over time or change in relation to another value. The value whose change the graph is illustrating is the vertical value, which is labeled to the right of the graph. The bottom line often represents the time period during which that the change is taking place.
As an example, the line graph to the right shows how the federal minimum wage has changed from 1940 to 2000. To find out how much the minimum wage was in a certain year, you just find that year on the graph and go straight up until you run into that blue line. From where you run into the blue line, look straight to the right or left and that’s what the minimum wage was that year.