To answer my own question thanks to @baptiste's tip-off, this is a device dependent issue due to RGui. If the image is saved directly out to file using png
everything works as intended. E.g.:
dev.new(width=1.3,height=0.3)
# repeat from here onwards only for png call below
par(mar=c(0,0,0,0))
set.seed(13)
x <- 1:10
y <- runif(10)
plot(x,y,type="n",xaxs="i",yaxs="i",ylim=c(0,1),bty="n")
polygon( c(1,x,max(x),0), c(0,y,0,0), col="lightblue", border=NA)
lines(x,y,lwd=1)
box(lwd=1)
Saving out to png from the "R Graphics" window gives my original stuffed up image:
Going directly to file using png
like:
png("textbox_direct.png",width=116,height=27)
# take code block from above
dev.off()
...gives the correct result: