Don't know what you are doing (helpful to show what you tried that didn't work), but your claim that cex.axis only affects the x-axis is not true:
set.seed(123)
foo <- data.frame(X = rnorm(10), Y = rnorm(10))
plot(Y ~ X, data = foo, cex.axis = 3)
at least for me with:
> sessionInfo()
R version 2.11.1 Patched (2010-08-17 r52767)
Platform: x86_64-unknown-linux-gnu (64-bit)
locale:
[1] LC_CTYPE=en_GB.UTF-8 LC_NUMERIC=C
[3] LC_TIME=en_GB.UTF-8 LC_COLLATE=en_GB.UTF-8
[5] LC_MONETARY=C LC_MESSAGES=en_GB.UTF-8
[7] LC_PAPER=en_GB.UTF-8 LC_NAME=C
[9] LC_ADDRESS=C LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_GB.UTF-8 LC_IDENTIFICATION=C
attached base packages:
[1] grid stats graphics grDevices utils datasets methods
[8] base
other attached packages:
[1] ggplot2_0.8.8 proto_0.3-8 reshape_0.8.3 plyr_1.2.1
loaded via a namespace (and not attached):
[1] digest_0.4.2 tools_2.11.1
Also, cex.axis affects the labelling of tick marks. cex.lab is used to control what R call the axis labels.
plot(Y ~ X, data = foo, cex.lab = 3)
but even that works for both the x- and y-axis.
Following up Jens' comment about using barplot(). Check out the cex.names argument to barplot(), which allows you to control the bar labels:
dat <- rpois(10, 3) names(dat) <- LETTERS[1:10] barplot(dat, cex.names = 3, cex.axis = 2)
As you mention that cex.axis was only affecting the x-axis I presume you had horiz = TRUE in your barplot() call as well? As the bar labels are not drawn with an axis() call, applying Joris' (otherwise very useful) answer with individual axis() calls won't help in this situation with you using barplot()
HTH