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.