The type of graph you use depends on the information you are trying to get from the experiment. If you are looking at the relationship between the diameter of the balloon versus the volume of the balloon, then plot the information as a line graph with the diameter on the horizontal axis and the volume on the vertical axis. If you are comparing the lung capacities of several different people, then you should use a bar graph. List each person's name on the horizontal axis and then plot the volume (lung capacity) on the vertical axis. You can then draw a rectangular bar over each person's name that goes up to their lung capacity value. If by pairs you mean that each person measured their lung capacity twice, then you should average the two values to get a single value.