Revision as of 23:25, 21 July 2010

Bad contour plot

Contour Plots

R has two built-in functions for contour plots, contour, and filled.contour. Examples of their use can be seen from the code below:

x <- 1:50
y <- 1:70
z <- matrix(expand.grid(x,y)$Var1^2 + expand.grid(x,y)$Var2^2,50,70)

# plain
contour(x,y,z) 

# adjusting levels
mylevels <- seq(0,7500,500) 
contour(x,y,z,levels=mylevels,xaxs='i',yaxs='i')

# filled contours
filled.contour(x,y,z,color.palette=heat.colors)
filled.contour(x,y,z,col=grey(seq(0,1,length=length(mylevels))))
# add this to line above and see bad match
contour(x,y,z,levels=mylevels,add=T)

However, it can be frustrating to realize that the way filled.contour was implemented does not allow removing the key or overplotting with lines and labels. To realize this, try the following:

filled.contour(x,y,z,col=grey(seq(0,1,length=length(mylevels))))
# add this to line above and see bad match
contour(x,y,z,levels=mylevels,add=T)

Good contour plot

A solution is to use the modified function, filled.contour2, as follows

# filled.contour function modified to not have key
filled.contour2(x,y,z,col=grey(seq(0.3,1,length=length(mylevels))))
# now we can successfully add lines and values
contour(x,y,z,levels=mylevels,add=T)

Here is the full modified function,

filled.contour2 <-
  function (x = seq(0, 1, length.out = nrow(z)),
            y = seq(0, 1, length.out = ncol(z)), z, xlim = range(x, finite = TRUE), 
            ylim = range(y, finite = TRUE), zlim = range(z, finite = TRUE), 
            levels = pretty(zlim, nlevels), nlevels = 20, color.palette = cm.colors, 
            col = color.palette(length(levels) - 1), plot.title, plot.axes, 
            key.title, key.axes, asp = NA, xaxs = "i", yaxs = "i", las = 1, 
            axes = TRUE, frame.plot = axes,mar, ...) 
{
  # modification by Ian Taylor of the filled.contour function
  # to remove the key and facilitate overplotting with contour()
  if (missing(z)) {
    if (!missing(x)) {
      if (is.list(x)) {
        z <- x$z
        y <- x$y
        x <- x$x
      }
      else {
        z <- x
        x <- seq.int(0, 1, length.out = nrow(z))
      }
    }
    else stop("no 'z' matrix specified")
  }
  else if (is.list(x)) {
    y <- x$y
    x <- x$x
  }
  if (any(diff(x) <= 0) || any(diff(y) <= 0)) 
    stop("increasing 'x' and 'y' values expected")
  mar.orig <- (par.orig <- par(c("mar", "las", "mfrow")))$mar
  on.exit(par(par.orig))
  w <- (3 + mar.orig[2]) * par("csi") * 2.54
  par(las = las)
  mar <- mar.orig
  plot.new()
  par(mar=mar)
  plot.window(xlim, ylim, "", xaxs = xaxs, yaxs = yaxs, asp = asp)
  if (!is.matrix(z) || nrow(z) <= 1 || ncol(z) <= 1) 
    stop("no proper 'z' matrix specified")
  if (!is.double(z)) 
    storage.mode(z) <- "double"
  .Internal(filledcontour(as.double(x), as.double(y), z, as.double(levels), 
                          col = col))
  if (missing(plot.axes)) {
    if (axes) {
      title(main = "", xlab = "", ylab = "")
      Axis(x, side = 1)
      Axis(y, side = 2)
    }
  }
  else plot.axes
  if (frame.plot) 
    box()
  if (missing(plot.title)) 
    title(...)
  else plot.title
  invisible()
}

See also

R tips