How could you plot more than one responding variable on a line graph

  use more than one line

Step-by-step explanation:

A line graph is not limited to a single line.

You can plot as many responding variables as you like. One way to do it is to use a separate line for each variable.

Another way to do it is to identify some relationship between the variables that may be of interest (their difference or ratio or some other function of the responding variables) and plot that.