contour {graphics}R Documentation

Display Contours

Description

Create a contour plot, or add contour lines to an existing plot.

Usage

contour(x, ...)

## Default S3 method:
contour(x = seq(0, 1, len = nrow(z)),
        y = seq(0, 1, len = ncol(z)),
        z,
        nlevels = 10, levels = pretty(zlim, nlevels), labels = NULL,
        xlim = range(x, finite = TRUE),
        ylim = range(y, finite = TRUE),
        zlim = range(z, finite = TRUE),
        labcex = 0.6, drawlabels = TRUE, method = "flattest",
        vfont = c("sans serif", "plain"),
        axes = TRUE, frame.plot = axes,
        col = par("fg"), lty = par("lty"), lwd = par("lwd"),
        add = FALSE, ...)

contourLines(x = seq(0, 1, len = nrow(z)),
             y = seq(0, 1, len = ncol(z)),
             z, nlevels = 10,
             levels = pretty(range(z, na.rm=TRUE), nlevels))

Arguments

x,y locations of grid lines at which the values in z are measured. These must be in ascending order. By default, equally spaced values from 0 to 1 are used. If x is a list, its components x$x and x$y are used for x and y, respectively. If the list has component z this is used for z.
z a matrix containing the values to be plotted (NAs are allowed). Note that x can be used instead of z for convenience.
nlevels number of contour levels desired iff levels is not supplied.
levels numeric vector of levels at which to draw contour lines.
labels a vector giving the labels for the contour lines. If NULL then the levels are used as labels.
labcex cex for contour labelling.
drawlabels logical. Contours are labelled if TRUE.
method character string specifying where the labels will be located. Possible values are "simple", "edge" and "flattest" (the default). See the Details section.
vfont if a character vector of length 2 is specified, then Hershey vector fonts are used for the contour labels. The first element of the vector selects a typeface and the second element selects a fontindex (see text for more information).
xlim, ylim, zlim x-, y- and z-limits for the plot.
axes, frame.plot logical indicating whether axes or a box should be drawn, see plot.default.
col color for the lines drawn.
lty line type for the lines drawn.
lwd line width for the lines drawn.
add logical. If TRUE, add to a current plot.
... additional graphical parameters (see par). The plot aspect ratio asp (see plot.window) and the arguments to title may also be supplied.

Details

contour is a generic function with only a default method in base R.

contourLines draws nothing, but returns a set of contour lines.

There is currently no documentation about the algorithm. The source code is in ‘$R_HOME/src/main/plot3d.c’.

The methods for positioning the labels on contours are "simple" (draw at the edge of the plot, overlaying the contour line), "edge" (draw at the edge of the plot, embedded in the contour line, with no labels overlapping) and "flattest" (draw on the flattest section of the contour, embedded in the contour line, with no labels overlapping). The second and third may not draw a label on every contour line.

For information about vector fonts, see the help for text and Hershey.

Notice that contour interprets the z matrix as a table of f(x[i], y[j]) values, so that the x axis corresponds to row number and the y axis to column number, with column 1 at the bottom, i.e. a 90 degree clockwise rotation of the conventional textual layout.

Value

contourLines returns a list of contours. Each contour is a list with elements:

level The contour level.
x The x-coordinates of the contour.
y The y-coordinates of the contour.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

filled.contour for “color-filled” contours, image and the graphics demo which can be invoked as demo(graphics).

Examples

x <- -6:16
op <- par(mfrow = c(2, 2))
contour(outer(x, x), method = "edge", vfont = c("sans serif", "plain"))
z <- outer(x, sqrt(abs(x)), FUN = "/")
## Should not be necessary:
z[!is.finite(z)] <- NA
image(x, x, z)
contour(x, x, z, col = "pink", add = TRUE, method = "edge",
        vfont = c("sans serif", "plain"))
contour(x, x, z, ylim = c(1, 6), method = "simple", labcex = 1)
contour(x, x, z, ylim = c(-6, 6), nlev = 20, lty = 2, method = "simple")
par(op)

## Persian Rug Art:
x <- y <- seq(-4*pi, 4*pi, len = 27)
r <- sqrt(outer(x^2, y^2, "+"))
opar <- par(mfrow = c(2, 2), mar = rep(0, 4))
for(f in pi^(0:3))
  contour(cos(r^2)*exp(-r/f),
          drawlabels = FALSE, axes = FALSE, frame = TRUE)

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)
par(opar); 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,
        vfont = c("sans serif", "plain"))
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)

## contourLines produces the same contour lines as contour
line.list <- contourLines(x, y, volcano)
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,
             vfont = c("sans serif", "plain"))
templines <- function(clines) {
  lines(clines[[2]], clines[[3]])
}  
invisible(lapply(line.list, templines))
par(opar)

[Package graphics version 2.1.0 Index]