Add different unique labels to each panel in lattice

Question

It's quite clear How to label panels in lattice using panel.text or ltext arguments. However, what if I want to use a different, unique label for each panel in lattice? Let me illustrate my point with this simplified Dotplot:

library(Hmisc)
#example data
data <- data.frame(transport=rep(c("bicycle","bus"),each=2), 
                   att=rep(c("behaviour control","intention"),2),
                   value=c(4.134,4.5,3.77,2.4), Lo=c(3.92,4.37,3.51,2.2),
                   Hi=c(4.34,4.62,4.02,2.61))
#labels I want to use
labels.hi=c("likely","easy")
labels.lo=c("unlikely","difficult")
#example dotplot
png("test.png",width=300, height=400)
Dotplot(transport ~ Cbind(value, Lo, Hi) | att, data, col=1,
        panel = function(x, y,...) {
          panel.Dotplot(x, y,...)
          ltext(2.5,1.5, labels=labels.lo)
          ltext(4.5,1.5, labels=labels.hi)
        })
dev.off()

This code gives me the plot below:

The upper panel gets a correct labels ("unlikely" and "likely"), but the lower panel just gets duplicate of the upper panel labels. Instead, I want to plot the remaining labels ("easy", "difficult") in the lower panel, but in the same location as upper panel.

I know I could define each label separately using ltext argument for every label, but it's quite impractical solution considering that my 'real-life' plot (heh) has more panels, and much more different unique labels. Any suggestions? Lattice only please.

Answer 1

(Nice to see a good lattice question.) I don't agree with agstudy that subscripts would be a good indexing strategy. In this case they work by accident because your points are the same number as your labels and in the same order. Subscripts are that mechanism for picking individual data points for panels rather than a mechanism for indexing panels. Consider using the packet.number() or panel.number() functions. In this instance I believe they return the same values, but consult their shared help page if you have more complex conditioning factors in mind:

Dotplot(transport ~ Cbind(value, Lo, Hi) | att, data, col=1,
        panel = function(x, y,...) {
          panel.Dotplot(x, y,...)
          if(packet.number()==1){ ltext(c(2.5,4.5) ,1.5, labels= labels.lo)}
          if(packet.number()==2){ ltext(c(2.5,4.5) ,1.5, labels=labels.hi)}
        })

If you had your labels in a matrix it would have been easy to use indexing with "[".

lab.mat=matrix(c( labels.hi,labels.lo), 2)
lab.mat
#     [,1]     [,2]       
#[1,] "likely" "unlikely" 
#[2,] "easy"   "difficult"

png("test.png",width=300, height=400)
Dotplot(transport ~ Cbind(value, Lo, Hi) | att, data, col=1,
        panel = function(x, y,...) {
          panel.Dotplot(x, y,...)
          { ltext(c(2.5,4.5) ,1.5, labels= lab.mat[packet.number(),])}
        })
dev.off()