Perhaps it is worth mentioning in: http://plplot.sourceforge.net/docbook-manual/plplot-html-5.9.8/postscript-driver.html
that the PostScript driver is able to output only EPS files (e.g. it has the EPS header hardcoded in line 245 of the drivers/ps.c)
... but it violates the EPS guidelines as the driver supports multi-page output which is not allowed in an EPS file
(c.f section 2 on page 7 in: http://partners.adobe.com/public/developer/en/ps/5002.EPSF_Spec.pdf\)