orientation of eps files

Yes, and it happens because by default R's plots have rotated text on the 
y axis, and it is this that tends to trigger auto-rotation.

The answer is in

https://www.stat.math.ethz.ch/pipermail/r-devel/2003-October/027766.html

various other items in the thread being off the point.

On Wed, 2 Mar 2005, Roger D. Peng wrote:

This is ghostscript feature, I believe.  See here:

https://www.stat.math.ethz.ch/pipermail/r-devel/2003-October/027759.html

I usually do

setenv GS_OPTIONS "-dAutoRotatePages=/None"

in tcsh.

-roger

Thanks to both of you for that clarification on this. It is curious that
I was able to restrict this behavior seemingly by adjusting the size of
the plot and the margins on the slide page when this behavior occurred.
Presumably those changes masked this underlying issue.

As an FYI, for those using the bash shell (typical for many Linuxen),
the commands would be:

GS_OPTIONS="-dAutoRotatePages=/None"
export GS_OPTIONS

or they can be combined in a single command:

export GS_OPTIONS="-dAutoRotatePages=/None"

which can be placed in a shell script or in ~/.bash_profile, which makes
the change "global" for the login session.

Thanks,

Marc