The plotted points are the cumulative scores based on zones at 1, 2, and 3 standard deviations from the center line.
The cumulative score is a sum of the weights of the plotted points. Each time the points cross the center line, the cumulative score is reset to zero. Each cumulative score helps to determine whether the process is in control. When the cumulative score is greater than or equal to 8, the process is out of control.