avoiding over-crowding of labels in r graphs

Question

I am working on avoid over crowding of the labels in the following plot:

set.seed(123)
position <- c(rep (0,5), rnorm (5,1,0.1), rnorm (10, 3,0.1), rnorm (3, 4, 0.2), 5, rep(7,5), rnorm (3, 8,2),  rnorm (10,9,0.5),
               rep (0,5), rnorm (5,1,0.1), rnorm (10, 3,0.1), rnorm (3, 4, 0.2), 5, rep(7,5), rnorm (3, 8,2),  rnorm (10,9,0.5))
group <- c(rep (1, length (position)/2),rep (2, length (position)/2)  )
mylab <- paste ("MR", 1:length (group), sep = "")
barheight <- 0.5

y.start <- c(group-barheight/2)
y.end <- c(group+barheight/2)
mydf <- data.frame (position, group, barheight, y.start, y.end, mylab)


plot(0,type="n",ylim=c(0,3),xlim=c(0,10),axes=F,ylab="",xlab="")
#Create two horizontal lines
require(fields)
yline(1,lwd=4)
yline(2,lwd=4)
#Create text for the lines
text(10,1.1,"Group 1",cex=0.7)
text(10,2.1,"Group 2",cex=0.7)
#Draw vertical bars
lng = length(position)/2
lg1 = lng+1
lg2 = lng*2
segments(mydf$position[1:lng],mydf$y.start[1:lng],y1=mydf$y.end[1:lng])
segments(mydf$position[lg1:lg2],mydf$y.start[lg1:lg2],y1=mydf$y.end[lg1:lg2])
text(mydf$position[1:lng],mydf$y.start[1:lng]+0.65, mydf$mylab[1:lng], srt = 90)
text(mydf$position[lg1:lg2],mydf$y.start[lg1:lg2]+0.65, mydf$mylab[lg1:lg2], srt = 90)

You can see some areas are crowed with the labels - when x value is same or similar. I want just to display only one label (when there is multiple label at same point). For example,

mydf$position[1:5] are all 0,

but corresponding labels mydf$mylab[1:5] -

 MR1  MR2  MR3  MR4  MR5 

I just want to display the first one "MR1".

Similarly the following points are too close (say the difference of 0.35), they should be considered a single cluster and first label will be displayed. In this way I would be able to get rid of overcrowding of labels. How can I achieve it ?

Answer 1

If you space the labels out and add some extra lines you can label every marker.

clpl <- function(xdata, names, y=1, dy=0.25, add=FALSE){
  o = order(xdata)
  xdata=xdata[o]
  names=names[o]
  if(!add)plot(0,type="n",ylim=c(y-1,y+2),xlim=range(xdata),axes=F,ylab="",xlab="")
  abline(h=1,lwd=4)
  dy=0.25
  segments(xdata,y-dy,xdata,y+dy)
  tpos = seq(min(xdata),max(xdata),len=length(xdata))
  text(tpos,y+2*dy,names,srt=90,adj=0)
  segments(xdata,y+dy,tpos,y+2*dy)
}

Then using your data:

clpl(mydf$position[lg1:lg2],mydf$mylab[lg1:lg2])

gives:

You could then think about labelling clusters underneath the main line.

I've not given much thought to doing multiple lines in a plot, but I think with a bit of mucking with my code and the add parameter it should be possible. You could also use colour to show clusters. I'm fairly sure these techniques are present in some of the clustering packages for R...

Obviously with a lot of markers even this is going to get smushed, but with a lot of clusters the same thing is going to happen. Maybe you end up labelling clusters with a this technique?