It would give users even more flexibility if they had the capability not only to
explicitly define the location of the gnuplot 'data' file (http://common-lisp.net/pipermail/clnuplot-devel/2009-February/000008.html)
but to also have the capability to
explicitly specify the location of the rendered (graphical) version of the chart/plot.

Something like

(defun fullname-rendered (plot type)
(let ((fullpath (fullpath plot)))
    (namestring (make-pathname
        ;; strip type info
        :name (pathname-name fullpath)
        :directory (pathname-directory fullpath)
        :type type))))

and then a modification of each 'set output' line for gnuplot (in the various WRITE-PLOT definitions) to something like

(format out "set output '~A'~C"
        (fullname-rendered plot "ps")
        #\Linefeed))

seems like it would do the trick...

- Alan