Easier way is to take the airport ISA temperature, 15C at sea level, add 39.4 degrees to it, (39.4 degrees Boeing)(39 degrees Embraer and Airbus)(35 degrees Bombardier), the resulting temperature is the maximum actual temperature that you can takeoff/land.
As for the chart, start on the bottom, lightest weight @ highest temperature, moving up you can see the weight decrease as the temperature decreases. This might make it easier for you to visualize.
Mutt