R help - Dec 2009 - Apparent different in symbol scaling between xyplot and grid.points

Dear R-users,

For the past few days, I have been trying to find the reason why some of 
my plots were showing symbols of different sizes, while I thought I was 
using the same .cex arguments everywhere. The problem is exemplified by 
the following example code where the xyplot and grid.points functions 
are used. The scaling factor is set to 0.84 in both the functions 
settings, but one can see that, in the pdf file, the blue symbols 
plotted by xyplot are smaller than the single black symbol created by 
grid.points. Playing with the trellis settings did not seem to solve the 
problem. I am missing a hidden scaling factor somewhere, but don't know 
where to look anymore

I would greatly appreciate the feedback of the list on this issue.

library(lattice)
library(grid)

pdf(file="test.pdf")

df <- data.frame(a=1:12,b=1:12,c=rep(1:4,each=3))

#trellis.par.set(superpose.symbol=list(cex=1))

xyplot(b~a|c,
       data=df,
       panel = function(x,y){
         panel.xyplot(x,y,pch=3,cex=0.84)}
       )

str(trellis.par.get())      
str(get.gpar()) 

grid.points(x=100,y=85,pch=3,gp=gpar(cex=0.84))

dev.off()

Hi,

I think the size mismatch occurs because of a different default for
the fontsize (and grid.points has a size of 1 character by default).
Compare the following two examples,

# default
grid.newpage()
pushViewport(viewport(x=unit(0.5, "npc"), y=unit(0.5,
"npc")))
lplot.xy(data.frame(x=0.55,y=0.5),type="p", pch=3)
grid.points(x=0.45,y=0.5, pch=3, gp=gpar(col="red"))


trellis.par.set("fontsize", list(points=12))

grid.newpage()
pushViewport(viewport(x=unit(0.5, "npc"), y=unit(0.5,
"npc")))
lplot.xy(data.frame(x=0.55,y=0.5),type="p", pch=3)
grid.points(x=0.45,y=0.5, pch=3, gp=gpar(col="red"))

HTH,

baptiste

2009/12/4 Sebastien Bihorel <Sebastien.Bihorel at
cognigencorp.com>:
> Dear R-users,
>
> For the past few days, I have been trying to find the reason why some of my
> plots were showing symbols of different sizes, while I thought I was using
> the same .cex arguments everywhere. The problem is exemplified by the
> following example code where the xyplot and grid.points functions are used.
> The scaling factor is set to 0.84 in both the functions settings, but one
> can see that, in the pdf file, the blue symbols plotted by xyplot are
> smaller than the single black symbol created by grid.points. Playing with
> the trellis settings did not seem to solve the problem. I am missing a
> hidden scaling factor somewhere, but don't know where to look anymore
>
> I would greatly appreciate the feedback of the list on this issue.
>
> library(lattice)
> library(grid)
>
> pdf(file="test.pdf")
>
> df <- data.frame(a=1:12,b=1:12,c=rep(1:4,each=3))
>
> #trellis.par.set(superpose.symbol=list(cex=1))
>
> xyplot(b~a|c,
> ? ? ?data=df,
> ? ? ?panel = function(x,y){
> ? ? ? ?panel.xyplot(x,y,pch=3,cex=0.84)}
> ? ? ?)
>
> str(trellis.par.get()) ? ? ?str(get.gpar())
> grid.points(x=100,y=85,pch=3,gp=gpar(cex=0.84))
>
> dev.off()
>
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