The Xbar chart plots the average of the measurements within each subgroup. The center line is the average of all subgroup averages. The control limits on the Xbar chart, which are set at a distance of 3 standard deviations above and below the center line, show the amount of variation that is expected in the subgroup averages.
Red points indicate subgroups that fail at least one of the tests for special causes and are not in control. If the same point fails multiple tests, then the point is labeled with the lowest test number to avoid cluttering the graph. If the chart shows out-of-control points, investigate those points.
Out-of-control points can influence the estimates of process parameters and prevent control limits from truly representing your process. If out-of-control points are due to special causes, then consider omitting these points from the calculations.
The Xbar chart shows that subgroup 3 failed test 1. When you hold the pointer over a red point, you can see more information about the subgroup.