[R] plot with two y axes

[Jim Lemon]

Thanks to Uwe Ligges, who found a couple of bugs in the function I
posted last week.  Here's a (hopefully) debugged version.

Jim

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plot2ax<-function(x1,y1,xlim1,ylim1,x2,y2,xlim2,ylim2,y2lab,type="b",pch1=1,pch2=2,...) {
 if(!missing(y1)) {
  oldmar<-par("mar")
  par(mar=c(5,4,4,4))
  if(missing(x1)) x1<-1:length(y1)
  if(missing(xlim1)) xlim1<-range(x1)
  if(missing(ylim1)) ylim1<-range(y1)
  plot(x1,y1,xlim=xlim1,ylim=ylim1,pch=pch1,type=type,...)
  if(!missing(y2)) {
   if(missing(x2)) x2<-1:length(y2)
   if(missing(xlim2)) xlim2<-range(x2)
   if(missing(ylim2)) ylim2<-range(pretty(y2))
   ax2val<-pretty(y2)
   xmult<-(xlim1[2] - xlim1[1])/(xlim2[2] - xlim2[1])
   ymult<-(ylim1[2] - ylim1[1])/(ylim2[2] - ylim2[1])
   axis(4,(ax2val-ylim2[1])*ymult+ylim1[1],labels=as.character(ax2val))
   points((x2-xlim2[1])*xmult+xlim1[1],(y2-ylim2[1])*ymult+ylim1[1],
    pch=pch2,type=type)
   if(!missing(y2lab)) mtext(y2lab,4,2)
  }
  par(oldmar)
 }
 else cat("Usage: plot2ax(x1,y1,xlim1,ylim1,x2,y2,xlim2,ylim2,y2lab,type=\"b\",pch1=1,pch2=2)\n")
}