showPsfrag is used to display (using ghostview) a postscript
image that contained psfrag LaTeX strings, by building a small LaTeX
script and running latex and dvips.
showPsfrag(filename)name or character string or character vector specifying file prefix.
Grant MC, Carlisle (1998): The PSfrag System, Version 3. Full
documentation is obtained by searching www.ctan.org for pfgguide.ps.
postscript, par, ps.options,
mgp.axis.labels, pdf,
trellis.device, setTrellis