Function to plot a segments plot copied and modified from kohonen package.

plot.centers.R

# Copying the kohonen:::plot.kohcodes routine, for equivalent kmeans stars diagram
plot.centers <- function (x, main) 
{
  palette.name <- terrain.colors

  codes <- x$codes
  nvars <- ncol(codes)
  maxlegendcols <- 3
  
  codeRendering <- "segments"
  
  margins <- rep(0.6, 4)
  
  if (!is.null(main)) 
    margins[3] <- margins[3] + 2
  
  par(mar = margins)
  
  if (codeRendering == "segments" & nvars < 15 & !is.null(colnames(codes))) {
    plot(x$grid, ylim = c(max(x$grid$pts[, 2]) + min(x$grid$pts[, 2]), -2))
    current.plot <- par("mfg")  # only needed when multiple plots
    plot.width <- diff(par("usr")[1:2])
    cex <- 1
    leg.result <- legend(x = mean(x$grid$pts[, 1]), xjust = 0.5, 
                         y = 0, yjust = 1, legend = colnames(codes), cex = cex, 
                         plot = FALSE, ncol = min(maxlegendcols, nvars), 
                         fill = palette.name(nvars))
    while (leg.result$rect$w > plot.width) {  # is it too wide?
      cex <- cex * 0.9
      leg.result <- legend(x = mean(x$grid$pts[, 1]), 
                           xjust = 0.5, y = 0, yjust = 1, legend = colnames(codes), 
                           cex = cex, plot = FALSE, ncol = min(maxlegendcols, nvars), 
                           fill = palette.name(nvars))
    }
    leg.result <- legend(x = mean(x$grid$pts[, 1]), xjust = 0.5, 
                         y = 0, yjust = 1, cex = cex, legend = colnames(codes), 
                         plot = FALSE, ncol = min(maxlegendcols, nvars), 
                         fill = palette.name(nvars))
    
    par(mfg = current.plot)
    plot(x$grid, ylim = c(max(x$grid$pts[, 2]) + min(x$grid$pts[, 2]), -leg.result$rect$h))
    
    legend(x = mean(x$grid$pts[, 1]), xjust = 0.5, y = 0, 
           yjust = 1, cex = cex, plot = TRUE, legend = colnames(codes), 
           ncol = min(maxlegendcols, nvars), fill = palette.name(nvars))
  }
  else {
    plot(x$grid)
  }
  
  title.y <- max(x$grid$pts[, 2]) + 1.2
  if (title.y > par("usr")[4] - 0.2) {
    title(main)
  }
  else {
    text(mean(range(x$grid$pts[, 1])), title.y, main, 
         adj = 0.5, cex = par("cex.main"), font = par("font.main"))
  }
  
  bgcol <- "transparent"
  
  symbols(x$grid$pts[, 1], x$grid$pts[, 2], 
          circles = rep(0.5, nrow(x$grid$pts)), inches = FALSE, add = TRUE, bg = bgcol)
  
  codemins <- apply(codes, 2, min)
  codes <- sweep(codes, 2, codemins)

  switch(codeRendering, segments = {
    stars(codes, locations = x$grid$pts, labels = NULL, 
          len = 0.4, add = TRUE, col.segments = palette.name(nvars), 
          draw.segments = TRUE)
  })
}