org-plot: Expand file name of output image

* lisp/org-plot.el (org-plot/gnuplot-script): The gnuplot process this
generated content is sent to may be operating in a different directory,
and hence this may write to a different file than intended.  By
expanding the file name first, this is no longer a concern.

lisp/org-plot.el

@@ -570,7 +570,7 @@ manner suitable for prepending to a user-specified script."
 			       org-plot/gnuplot-term-extra
 			     (funcall org-plot/gnuplot-term-extra type))))
       (when file ; output file
-	(funcall ats (format "set output '%s'" file)))
+	(funcall ats (format "set output '%s'" (expand-file-name file))))
 
       (when plot-pre
 	(funcall ats (funcall plot-pre table data-file num-cols params plot-str)))