[R] Different behavior of mtext

[Sébastien]

Dear R Users,

I am quite surprised to see that mtext gives different results when it 
is used with 'pairs' and with "plot'. In the two following codes, it 
seems that the 'at' argument in mtext doesn't consider the same unit system.

I would appreciate your comments on this issue.

Sebastien

##### Pairs

mydata<-data.frame(x=1:10,y=1:10)

par(cex.main=1, cex.axis=1, cex.lab=1, lwd=1,
    mar=c(5 + 5,4,4,2)+0.1)
   
pairs(mydata,oma=c(5 + 5,4,4,2))

mylegend<-c("mylegend A","mylegend B","mylegend C","mylegend test")
mylegend.width = strwidth(mylegend[which.max(nchar(mylegend))], "figure")

for (i in 1:4) {
mtext(text=mylegend[i],
        side = 1,
        line = 3+i,
        at = unit((1-mylegend.width)/2,"npc"),            # centers the 
legend at the bottom
        adj=0,
        padj=0)}

##### plot

mydata<-data.frame(x=1:10,y=1:10)

par(cex.main=1, cex.axis=1, cex.lab=1, lwd=1,
    mar=c(5 + 5,4,4,2)+0.1)
   
plot(mydata,oma=c(5 + 5,4,4,2))

mylegend<-c("mylegend A","mylegend B","mylegend C","mylegend test")
mylegend.width = strwidth(mylegend[which.max(nchar(mylegend))], "figure")

for (i in 1:4) {
mtext(text=mylegend[i],
        side = 1,
        line = 3+i,
        at = unit((1-mylegend.width)/2,"npc"),            # should 
center the legend at the bottom but doesn't do it !
        adj=0,
        padj=0)}