R help - Sep 2012 - Plot to tiff, font size problem in multiple plot figures

I am creating graphs for a publication and would like them to have the same
font size... but when I create a figure with multiple plots, the font size
decreases even though I haven't changed the tiff() resolution or
pointsize specifications, I have increased the figure size according to how
many plots it will ultimately have, and I have made sure the margins are
equivalent for single and multiple plot figures... example code below (font
size is consistent between 1x1 and 2x1 figure but decreases for 3x2 figure):

tiff("1x1.tif",
width=3,height=2.5,units="in",res=600,pointsize=8,compression="lzw",restoreConsole=T)
par(mfrow=c(1,1),mar=c(4,4,.5,.5)+0.1)
plot(x=rnorm(10),y=rnorm(10))
dev.off()

tiff("2x1.tif",
height=2.5*2,width=3,units="in",res=600,pointsize=8,compression="lzw",restoreConsole=T)
par(mfrow=c(2,1),mar=c(2,4,2.5,0.5)+0.1)
plot(x=rnorm(10),y=rnorm(10),xaxt="n",xlab="")
par(mar=c(4,4,0.5,0.5)+0.1)
plot(x=rnorm(10),y=rnorm(10))
dev.off()

tiff("3x2.tif",
height=2.5*3,width=3*2,units="in",res=600,pointsize=8,compression="lzw",restoreConsole=T)
par(mfrow=c(3,2),mar=c(.5,4,4,0.5)+0.1)
plot(x=rnorm(10),y=rnorm(10),xaxt="n",xlab="")
par(mar=c(.5,2,4,2.5)+0.1)
plot(x=rnorm(10),y=rnorm(10),xaxt="n",xlab="",yaxt="n",ylab="")
par(mar=c(2.5,4,2,0.5)+0.1)
plot(x=rnorm(10),y=rnorm(10),xaxt="n",xlab="")
par(mar=c(2.5,2,2,2.5)+0.1)
plot(x=rnorm(10),y=rnorm(10),xaxt="n",xlab="",yaxt="n",ylab="")
par(mar=c(4.5,4,0,0.5)+0.1)
plot(x=rnorm(10),y=rnorm(10))
par(mar=c(4.5,2,0,2.5)+0.1)
plot(x=rnorm(10),y=rnorm(10),yaxt="n",ylab="")
dev.off()

Why is this happening?
Any help greatly appreciated.
claudia

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