The diagram is just a large circle with lost of lines radiating from the centre. To assess your field of vision the use a machine. It consists of a large screen shaped like a saucer. In the centre of it there is a pin dot light. There is a chin rest which you rest your chin on and one of your eyes are covered. Keeping you open eye focused on the centre you have a button in your hand which you press when you see a light anywhere on the screen. When you press the button they plot the position on the circle diagram. They repeat the same for the other eye. All in all it takes maybe 10 minutes. And then when you see the diagrams you connect the dots and see how good your sight is.