Sometimes you are dealing with more than one graph.

If you can express two variables into the same units and if they both depend on the same variable, you can draw their graphs in one figure

• In a point of intersection of both graphs the variables have the same value on the vertical axis.

• You can make a value added graph by adding together the two corresponding values.

• You can make a value subtracted graph by subtracting the corresponding values from each other.

You can also have the situation that two graphs have one common variable. For instance, in one graph the variable $y$ depends on $x$ and in the other graph $x$ depends on t. You can now chain the variables: for a value of $t$ you find a value of $x$ and that gives you the corresponding value of y.