# axis.break places a break marker at the position "breakpos" 
# in user coordinates on the axis nominated - see axis().

axis.break<-function(axis=1,breakpos,bgcol="white",breakcol="black",
 style=c("slash","zigzag"),brw=0.02) {
 
 if(!missing(breakpos)) {
  # get the coordinates of the outside of the plot
  figxy<-par("usr")
  # flag if either axis is logarithmic
  xaxl<-par("xlog")
  yaxl<-par("ylog")
  # calculate the x and y offsets for the break
  xw<-(figxy[2]-figxy[1])*brw
  yw<-(figxy[4]-figxy[3])*brw
  if(xaxl && (axis == 1 || axis == 3)) breakpos<-log10(breakpos)
  if(yaxl && (axis == 2 || axis == 4)) breakpos<-log10(breakpos)
  # set up the "blank" rectangle (left, bottom, right, top)
  switch(axis,
   br<-c(breakpos-xw/2,figxy[3]-yw/2,breakpos+xw/2,figxy[3]+yw/2),
   br<-c(figxy[1]-xw/2,breakpos-yw/2,figxy[1]+xw/2,breakpos+yw/2),
   br<-c(breakpos-xw/2,figxy[4]-yw/2,breakpos+xw/2,figxy[4]+yw/2),
   br<-c(figxy[2]-xw/2,breakpos-yw/2,figxy[2]+xw/2,breakpos+yw/2),
   stop("Improper axis specification."))
  # get the current setting of xpd
  old.xpd<-par("xpd")
  # don't cut the break off at the edge of the plot
  par(xpd=T)
  # correct for logarithmic axes
  if(xaxl) br[c(1,3)]<-10^br[c(1,3)]
  if(yaxl) br[c(2,4)]<-10^br[c(2,4)]
  # draw the "blank" rectangle
  rect(br[1],br[2],br[3],br[4],col=bgcol,border=bgcol)
  if(style == "slash") {
   # calculate the slash ends
   if(axis == 1 || axis == 3) {
    xbegin<-c(breakpos-xw,breakpos)
    xend<-c(breakpos,breakpos+xw)
    ybegin<-c(br[2],br[2])
    yend<-c(br[4],br[4])
    if(xaxl) {
     xbegin<-10^xbegin
     xend<-10^xend
    }
   }
   else {
    xbegin<-c(br[1],br[1])
    xend<-c(br[3],br[3])
    ybegin<-c(breakpos-yw,breakpos)
    yend<-c(breakpos,breakpos+yw)
    if(yaxl) {
     ybegin<-10^ybegin
     yend<-10^yend
    }
   }
  }
  else {
   # calculate the zigzag ends
   if(axis == 1 || axis == 3) {
    xbegin<-c(breakpos-xw/2,breakpos-xw/4,breakpos+xw/4)
    xend<-c(breakpos-xw/4,breakpos+xw/4,breakpos+xw/2)
    ybegin<-c(ifelse(yaxl,10^figxy[3+(axis==3)],figxy[3+(axis==3)]),br[4],br[2])
    yend<-c(br[4],br[2],ifelse(yaxl,10^figxy[3+(axis==3)],figxy[3+(axis==3)]))
    if(xaxl) {
     xbegin<-10^xbegin
     xend<-10^xend
    }
   }
   else {
    xbegin<-c(ifelse(xaxl,10^figxy[1+(axis==4)],figxy[1+(axis==4)]),br[1],br[3])
    xend<-c(br[1],br[3],ifelse(xaxl,10^figxy[1+(axis==4)],figxy[1+(axis==4)]))
    ybegin<-c(breakpos-yw/2,breakpos-yw/4,breakpos+yw/4)
    yend<-c(breakpos-yw/4,breakpos+yw/4,breakpos+yw/2)
    if(yaxl) {
     ybegin<-10^ybegin
     yend<-10^yend
    }
   }
  }
  # draw the segments
  segments(xbegin,ybegin,xend,yend,col=breakcol,lty=1)
  # restore xpd
  par(xpd=old.xpd)
 }
 else {
  cat("Usage: axis.break(axis=1,breakpos,bgcol=\"white\",breakcol=\"black\",\n")
  cat("\tstyle=c(\"slash\",\"zigzag\"),brw=0.02)\n")
 }
}