The line graph report offers you the Multi-axis option, this is useful when you want to plot two or more different variables in the same graph with a large value range. The Multiaxis let you have one vertical axe on the plot for each measurement unit.
In the following graph the temperature and the relative humidity are plotted, note how different are the values of each line from the other that makes difficult to note the changes on the values of each line.
After selecting the Multiaxis option, the vertical axis will change and the value range on each axis will be fixed according to the maximum and minimum value of each line. So now on the left side of the graph the Celsius degree axis is shown and on the right the relative humidity percentage is shown. That makes easier to note how both variables are related and the small changes on each one.