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Problem with contour()

Helmut Schütz 
Dear all,

I can't get my head around how contour lines are drawn.
Working example(x and y are parameters of a certain test and z the 
resulting power):

library(PowerTOST)
x??? <- 0.90
y??? <- 0.35
res? <- as.numeric(sampleN.TOST(theta0 = x, CV = y, design = "2x2x4",
 ??????????????????????????????? method = "central", details = FALSE,
 ??????????????????????????????? print = FALSE)[7:8])
mesh <- 28
ys?? <- unique(sort(c(y, seq(y*.8, y*1.2, length.out = mesh))))
xs?? <- unique(sort(c(x, seq(x*0.95, 1, length.out = mesh))))
z??? <- matrix(nrow = length(ys), ncol = length(xs),
 ?????????????? dimnames = list(paste0("y.", signif(ys, 5)),
 ?????????????????????????????? paste0("x.", signif(xs, 5))))
for (i in seq_along(ys)) {
 ? for (j in seq_along(xs)) {
 ??? z[i, j] <- suppressMessages(
 ???????????????? power.TOST(CV = ys[i], theta0 = xs[j], design = "2x2x4",
 ??????????????????????????? method = "central", n = res[1]))
 ? }
}
z??? <- z[nrow(z):1, ncol(z):1]????????? # reverse rows & columns
z[paste0("y.", y), paste0("x.", x)] == res[2] # should be TRUE
contour(xs, ys, z, nlevels = 20, las = 1, labcex = 1,
 ??????? xlab = "x", ylab = "y", main = paste("n =", n))
abline(h = y, v = x, lty = 2)
points(x, y, col = "red", cex = 1.5)
text(x, y, labels = signif(z[paste0("y.", y), paste0("x.", x)], 6),
 ???? col = "red", adj = c(-0.1, 1.5))

At x = 0.9 and y = 0.35 z = 0.8130092. Obviously this does not agree 
with the contour-lines.
I'm sure that I screwed up - but where?

All the best,
Helmut