# [Rd] Fwd: [R] size of point symbols

Prof Brian Ripley ripley at stats.ox.ac.uk
Tue May 26 08:35:42 CEST 2009

```I don't know where you get your claims from.  R graphics is handled
internally in inches, with a device-specific mapping to pixels/points
etc (which is documented for each device on its help page).  This has
to be done carefully, as pixels may not be square.

What the meaning of pch=1:23 is in terms of coordinates is not
documented except via the sources.  The source is function GESymbol in
file src/main/engine.c, so for example pch = 2 is

case 2:	/* S triangle - point up */
r = toDeviceHeight(TRC0 * xc, GE_INCHES, dd);
yc = toDeviceHeight(TRC2 * xc, GE_INCHES, dd);
xc = toDeviceWidth(TRC1 * xc, GE_INCHES, dd);
xx[0] = x; yy[0] = y+r;
xx[1] = x+xc; yy[1] = y-yc;
xx[2] = x-xc; yy[2] = y-yc;
gc->fill = R_TRANWHITE;
GEPolygon(3, xx, yy, gc, dd);
break;

which as you see is in inches, not mm as you asserted.  The first line
sets xc to 0.375 inches for cex=1, for example.

You need to take the stroke width (as set by lty) into account when
assessing the visual size of symbols

On Mon, 25 May 2009, baptiste auguie wrote:

> 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).

You are expected to study the sources for yourself.  That's part of
the price of R.

There is a manual, 'R Internals', that would have explained to you
that graphics.c is part of base graphics and hence not of grid
graphics.

> 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 Auguié
>
> School of Physics
> University of Exeter
> Exeter, Devon,
> EX4 4QL, UK
>
> Phone: +44 1392 264187
>
> http://newton.ex.ac.uk/research/emag
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

--
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595
```