The method for building up the diagrams is based on a phasor diagram as a representation of a sinusoidal wave.  Figure 5 shows the phasor diagram on the left where time travels anticlockwise, and the associated sinusoidal waveform on the right where time travels along the positive absissa, for 12 samples in the period.  Notice that there are in fact 13 sample points, because the 0° (the initial condition) is included as a sample, which is in the same place on the phasor diagram as the sample at 360°.