Your line graph labels are the things that you want to compare. Quantity and time are the most common, but you can use anything.
If you want to see how the price of something changed over time, then time and quantity are your choice for axis labels.
If you wanted to know how much flour to use to make cookies for any number of people, you could just multiply it out. You could also create a line graph that you could refer to. You would just decide how many people you want to make cookies for and refer to your graph to find out how much flour you would need. The graph labels for the axes would be number of cookies and amount of flour.
