The plotted points on the I chart are the subgroup means.
The plotted points on the MR chart are the moving ranges of the subgroup means. The moving range is the absolute value of the difference between two or more consecutive points.
The plotted points on the R chart represent the subgroup ranges, and the plotted points on a S chart are the subgroup standard deviations.
If the process is in control, the points vary randomly around the center line, and the process exhibits only common-cause variation. Investigate points that fall outside the control limits or that exhibit nonrandom patterns for possible special-cause variation.