Inverting polygon

An even better solution, obviating the extra call to lines is:

n <- 50 * 3
x <- runif (n)
y <- runif (n)
x[(seq(n) %% 3) == 0] <- NA
y[(seq(n) %% 3) == 0] <- NA

plot.new ()
range.x <- range (x, na.rm=TRUE)
range.y <- range (y, na.rm=TRUE)
plot.window (range.x, range.y, asp=1)
lines (x, y)

wind.x <- c(range.x, rev (range.x), range.x[1])
wind.y <- c(range.y[1], range.y, rev (range.y))

# example polygon
poly.x <- wind.x / 2 + 0.25
poly.y <- wind.y / 2 + 0.25

mask.x <- c(wind.x, poly.x)
mask.y <- c(wind.y, poly.y)
polygon (mask.x, mask.y, col="white", border="white")