Each graph has two lines, one for each mic, in different colours. The curve for each mic represents the sensitivity in each horizontal direction - the distance from the centre indicating the sensitivity (you will come across graphs with linear and with logarithmic scales for this - the linked ones are logarithmic, as indicated by the dB notation). Of course, in real life, the graphs should be three-dimensional, and rotating the graph about a front/back axis (for the individual microphone) will generate the appropriate solid shape.
Paul