This function computes control limits for the x-charts, optionally revising them in the case that individual values plot outside the trial control limits.
xCC(x, sigma, k = 3, mu, newdata)a numeric vector consisting of the subgroup averages
the control chart parameter governing the width of the control limits
an estimate of the in-control standard deviation
(optional) an estimate of the in-control mean
(optional) new observations
a list consisting of the upper and lower control limits and the centerline.
Montgomery, D.C. (2000) Introduction to Statistical Quality Control, Wiley
plot.CC