How could you plot more than one responding (dependent) variable on a line graph?
There are several ways you could do this, depending on whether or not the dependent variables are of the same order of magnitude or not. If all your y-values are relatively the same you could show different sets of data by using different shapes for the data points and/or different colors for the data points and the corresponding lines. You just need to add a legend to explain what the shapes and/or colors correspond to in your data sets.
If you have two sets of dependent data that differ significantly in magnitude, you could have one set of values on the left vertical axis and a second set of values on a right vertical axis. Again, use different shapes and/or colors along with a legend, to denote which colors go with the left axis values and which go with the right axis values.
And of course, be sure to label all your axes with the appropriate names and values of the data points.