Read the graph to determine the initial values. For example, if you are determining how much it costs to produce 100 products, look up 100 on the X-axis (the horizontal axis of the graph, usually labeled quantity). Trace a vertical line up to the graph, and read the value on the Y-axis (the vertical line, usually labeled cost per unit).
Find the second point on the graph. For example, look up 150 on the X-axis. Trace a line up to the graph, and read the value on the Y-axis.
Subtract the two Y-values you found in Steps 1 and 2 to find the change in cost per unit. For example, if you found Y-values of 50 for the initial value and 75 for the second value, then:
75 -- 50 = 25.
25 is the increase in cost per unit for your cost curve.
Subtract the two X-values you used in Step 1 and 2 to find the difference in quantity. For example, if you used 150 and 100, then:
150 -- 100 = 50.
So 50 is the increase in quantity for your cost curve.