Answer:
A line graph is also used when there are two variables—but only makes sense when you're tracking changes over a continuous measurement (e.g., time, temperature, distance). Line graphs are especially useful for illustrating direction (i.e., peaks and dips) rather than magnitude.
Line graphs are used to track changes over short and long periods of time. When smaller changes exist, line graphs are better to use than bar graphs. Line graphs can also be used to compare changes over the same period of time for more than one group.