size of point symbols

Dear all,


Having received no answer in r-help I'm trying r-devel (hoping this is  
not a stupid question).

I don't understand the rationale behind the absolute sizes of the point
symbols, and I couldn't find it documented (I got lost in the C code  
graphics.c and gave up). The example below uses
Grid to check the size of the symbols against a square of 10mm x 10mm.

checkOneSymbol <- function(pch=0){
   gTree(children=gList(
       rectGrob(0.5, 0.5, width=unit(10, "mm"), height=unit(10,
"mm"),
               gp=gpar(lty=2, fill=NA, col=alpha("black", 0.5))),
   pointsGrob(0.5, 0.5, size=unit(10, "mm"),pch=pch,
       gp=gpar(col=alpha("red", 0.5)))
   ))

}
all.symbols <- lapply(0:23, checkOneSymbol)

pdf("symbols.pdf", height=1.2/2.54, width=24.2/2.54)

vp <- viewport(width=0.5, height=0.5, name="main")
pushViewport(vp)

pushViewport(viewport(layout=grid.layout(1, 24,
                       widths=unit(10, "mm"),
                       heights=unit(10, "mm"),
                       just="center")))

for(ii in 0:23){
pushViewport(viewport(layout.pos.col=ii+1, layout.pos.row=1))
grid.draw(all.symbols[[ii+1]])
upViewport(1)
}
dev.off()

What dictates the size of each symbol? (in other words, why is pch=21
a circle of radius given in inches, while pch=2 is a triangle of base
length specified  in mm and offset vertically?, etc.)

I'm trying to develop a new symbol for the ggplot2 package where the  
size is to be accurately mapped onto the data either in linear size or  
area. I was expecting a similar idea behind the choice of base  
symbols. Is this documented?

Best regards,

baptiste

_____________________________

Baptiste Augui?

School of Physics
University of Exeter
Stocker Road,
Exeter, Devon,
EX4 4QL, UK

Phone: +44 1392 264187

http://newton.ex.ac.uk/research/emag