TITLE(contour @@ Display contours )
USAGE(
contour(x=, y=, z, nlevels = 10,
        levels = pretty(range(z, na.rm = TRUE), nlevels),
        labcex = 0, col = par("fg"), lty = par("lty"), add = FALSE)
)
ALIAS(contour)
ARGUMENTS(
ARG(x,y @@ locations of grid lines at which the values in LANG(z)
are measured.  These must be in ascending order.
By default, equally spaced values from 0 to 1 are used.)
ARG(z @@ a matrix containing the values to be plotted (LANG(NA)s are allowed).)
ARG(nlevels @@ number of contour levels desired BOLD(iff)
LANG(levels) is not supplied.) 
ARG(levels @@ numeric vector of levels at which to draw contour lines.)
ARG(labcex @@ LANG(cex) for contour labelling. (not yet used))
ARG(col @@ color for the lines drawn )
ARG(lty @@ line type for the lines drawn  )
ARG(add @@ logical. if LANG(TRUE), add to a current plot.)
)
DESCRIPTION(
Draws contour lines for the desired levels.
There is currently no documentation about the algorithm.
The source code is currently in ITALIC(\$RHOME/src/library/main/plot3d.c).
)
SEEALSO(
LANG(LINK(image)) and the extensive example in

LANG(system.file("demos/graphics","graphics")).
)
EXAMPLES(
data("volcano")
rx <- range(x <- 10*1:nrow(volcano))
ry <- range(y <- 10*1:ncol(volcano))
ry <- ry + c(-1,1)*(diff(rx)-diff(ry))/2
tcol <- terrain.colors(12)
opar <- par(pty="s", bg="lightcyan")
plot(x=0,y=0,type='n', xlim=rx,  ylim=ry, xlab='', ylab='')
u <- par("usr")
rect(u[1],u[3], u[2],u[4], col = tcol[8], border='red')
contour(x, y, volcano, col=tcol[2], lty="solid", add=TRUE)
title("A Topographic Map of Maunga Whau",font=4)
abline(h=200*0:4,v=200*0:4, col= 'lightgray', lty=2, lwd=0.1)
)
