One way to go is to adjust the original plot.ewma.qcc function which you can find here.
With a bit of effort you can work out how the limits are handled in this function. You could create your own plot.ewma.qcc2 function as shown below.
The function is lengthy but halfway through, below the comment add more control limits you will see I added four lines() function calls to calculate and display the 1 and 2 sigma control lines.
Here's how this looks like:
plot.ewma.qcc2 <- function (x, add.stats = TRUE, chart.all = TRUE, label.limits = c("LCL",
"UCL"), title, xlab, ylab, ylim, axes.las = 0, digits = getOption("digits"),
restore.par = TRUE, ...)
{
object <- x
if ((missing(object)) | (!inherits(object, "ewma.qcc")))
stop("an object of class `ewma.qcc' is required")
type <- object$type
data.name <- object$data.name
center <- object$center
std.dev <- object$std.dev
stats <- object$statistics
limits <- object$limits
newstats <- object$newstats
newdata.name <- object$newdata.name
violations <- object$violations
nviolations <- length(violations)
if (chart.all) {
statistics <- c(stats, newstats)
indices <- 1:length(statistics)
}
else {
if (is.null(newstats)) {
statistics <- stats
indices <- 1:length(statistics)
}
else {
statistics <- newstats
indices <- seq(length(stats) + 1, length(stats) +
length(newstats))
}
}
if (missing(title)) {
if (is.null(newstats))
main.title <- paste("EWMA Chart\nfor", data.name)
else if (chart.all)
main.title <- paste("EWMA Chart\nfor", data.name,
"and", newdata.name)
else main.title <- paste("EWMA Chart\nfor", newdata.name)
}
else main.title <- paste(title)
oldpar <- par(no.readonly = TRUE)
if (restore.par)
on.exit(par(oldpar))
mar <- pmax(oldpar$mar, c(4.1, 4.1, 3.1, 2.1))
par(bg = qcc.options("bg.margin"), cex = oldpar$cex *
qcc.options("cex"), mar = if (add.stats)
pmax(mar, c(7.6, 0, 0, 0))
else mar)
plot(indices, statistics, type = "n", ylim = if (!missing(ylim))
ylim
else range(statistics, limits), ylab = ifelse(missing(ylab),
"Group Summary Statistics", ylab), xlab = ifelse(missing(xlab),
"Group", xlab), axes = FALSE)
rect(par("usr")[1], par("usr")[3], par("usr")[2],
par("usr")[4], col = qcc.options("bg.figure"))
axis(1, at = indices, las = axes.las, labels = if (is.null(names(statistics)))
as.character(indices)
else names(statistics))
axis(2, las = axes.las)
box()
top.line <- par("mar")[3] - length(capture.output(cat(main.title)))
top.line <- top.line - if (chart.all & (!is.null(newstats)))
0.1
else 0.5
mtext(main.title, side = 3, line = top.line, font = par("font.main"),
cex = qcc.options("cex"), col = par("col.main"))
abline(h = center, lty = 1)
lines(indices, limits[indices, 1], lty = 2)
lines(indices, limits[indices, 2], lty = 2)
# add more control limits
lines(indices, center + object$sigma, lty = 2, col = "red")
lines(indices, center - object$sigma, lty = 2, col = "red")
lines(indices, center + 2*object$sigma, lty = 2, col = "green")
lines(indices, center - 2*object$sigma, lty = 2, col = "green")
points(indices, statistics, pch = 3, cex = 0.8)
lines(indices, object$y[indices], type = "o", pch = 20)
mtext(label.limits, side = 4, at = limits[nrow(limits), ],
las = 1, line = 0.1, col = gray(0.3), cex = par("cex"))
if (nviolations > 0) {
if (is.null(qcc.options("beyond.limits")))
stop(".qcc.options$beyond.limits undefined. See help(qcc.options).")
points(violations, object$y[violations], col = qcc.options("beyond.limits")$col,
pch = qcc.options("beyond.limits")$pch)
}
if (chart.all & !is.null(newstats)) {
len.obj.stats <- length(stats)
len.new.stats <- length(newstats)
abline(v = len.obj.stats + 0.5, lty = 3)
mtext("Calibration data", cex = par("cex") *
0.8, at = len.obj.stats/2, line = 0, adj = 0.5)
mtext("New data", cex = par("cex") * 0.8,
at = len.obj.stats + len.new.stats/2, line = 0, adj = 0.5)
}
if (add.stats) {
plt <- par()$plt
usr <- par()$usr
px <- diff(usr[1:2])/diff(plt[1:2])
xfig <- c(usr[1] - px * plt[1], usr[2] + px * (1 - plt[2]))
at.col <- xfig[1] + diff(xfig[1:2]) * c(0.15, 0.6)
top.line <- 4.5
mtext(paste("Number of groups = ", length(statistics),
sep = ""), side = 1, line = top.line, adj = 0,
at = at.col[1], font = qcc.options("font.stats"),
cex = par("cex") * qcc.options("cex.stats"))
if (length(center) == 1) {
mtext(paste("Center = ", signif(center[1],
digits = digits), sep = ""), side = 1,
line = top.line + 1, adj = 0, at = at.col[1],
font = qcc.options("font.stats"), cex = par("cex") *
qcc.options("cex.stats"))
}
else {
mtext("Center is variable", side = 1, line = top.line +
1, adj = 0, at = at.col[1], font = qcc.options("font.stats"),
cex = par("cex") * qcc.options("cex.stats"))
}
mtext(paste("StdDev = ", signif(std.dev, digits = digits),
sep = ""), side = 1, line = top.line + 2, adj = 0,
at = at.col[1], font = qcc.options("font.stats"),
cex = par("cex") * qcc.options("cex.stats"))
mtext(paste("Smoothing parameter = ", signif(object$lambda,
digits = digits)), side = 1, line = top.line, adj = 0,
at = at.col[2], font = qcc.options("font.stats"),
cex = par("cex") * qcc.options("cex.stats"))
mtext(paste("Control limits at ", object$nsigmas,
"*sigma", sep = ""), side = 1, line = top.line +
1, adj = 0, at = at.col[2], font = qcc.options("font.stats"),
cex = par("cex") * qcc.options("cex.stats"))
mtext(paste("No. of points beyond limits =", nviolations),
side = 1, line = top.line + 2, adj = 0, at = at.col[2],
font = qcc.options("font.stats"), cex = par("cex") *
qcc.options("cex.stats"))
}
invisible()
}
