ob-R.el: allow use of alternative graphics devices

* lisp/ob-R.el (org-babel-R-graphics-devices): New defvar.
(org-babel-R-construct-graphics-device-call): Use it instead of a
hard-coded list of graphics devices.

The `org-babel-R-graphics-devices' variable now holds the association
between graphics file types and R functions.  This allows users by
modifying this alist to choose to use e.g. the Cairo* functions from the
R package “Cairo” or one of several alternative devices for SVG
output (SVGAnnotation, RSVGTipsDevice).

lisp/ob-R.el

@@ -234,31 +234,40 @@ current code buffer."
   (and (member "graphics" (cdr (assq :result-params params)))
        (cdr (assq :file params))))
 
+(defvar org-babel-R-graphics-devices
+  '((:bmp "bmp" "filename")
+    (:jpg "jpeg" "filename")
+    (:jpeg "jpeg" "filename")
+    (:tikz "tikz" "file")
+    (:tiff "tiff" "filename")
+    (:png "png" "filename")
+    (:svg "svg" "file")
+    (:pdf "pdf" "file")
+    (:ps "postscript" "file")
+    (:postscript "postscript" "file"))
+  "An alist mapping graphics file types to R functions.
+
+Each member of this list is a list with three members:
+1. the file extension of the graphics file, as an elisp :keyword
+2. the R graphics device function to call to generate such a file
+3. the name of the argument to this function which specifies the
+   file to write to (typically \"file\" or \"filename\")")
+
 (defun org-babel-R-construct-graphics-device-call (out-file params)
   "Construct the call to the graphics device."
-  (let ((devices
-	 '((:bmp . "bmp")
-	   (:jpg . "jpeg")
-	   (:jpeg . "jpeg")
-	   (:tikz . "tikz")
-	   (:tiff . "tiff")
-	   (:png . "png")
-	   (:svg . "svg")
-	   (:pdf . "pdf")
-	   (:ps . "postscript")
-	   (:postscript . "postscript")))
-	(allowed-args '(:width :height :bg :units :pointsize
-			       :antialias :quality :compression :res
-			       :type :family :title :fonts :version
-			       :paper :encoding :pagecentre :colormodel
-			       :useDingbats :horizontal))
-	(device (and (string-match ".+\\.\\([^.]+\\)" out-file)
-		     (match-string 1 out-file)))
-	(extra-args (cdr (assq :R-dev-args params))) filearg args)
-    (setq device (or (and device (cdr (assq (intern (concat ":" device))
-					    devices))) "png"))
-    (setq filearg
-	  (if (member device '("pdf" "postscript" "svg" "tikz")) "file" "filename"))
+  (let* ((allowed-args '(:width :height :bg :units :pointsize
+				:antialias :quality :compression :res
+				:type :family :title :fonts :version
+				:paper :encoding :pagecentre :colormodel
+				:useDingbats :horizontal))
+	 (device (and (string-match ".+\\.\\([^.]+\\)" out-file)
+		      (match-string 1 out-file)))
+	 (device-info (or (assq (intern (concat ":" device))
+				org-babel-R-graphics-devices)
+                          (assq :png org-babel-R-graphics-devices)))
+        (extra-args (cdr (assq :R-dev-args params))) filearg args)
+    (setq device (nth 1 device-info))
+    (setq filearg (nth 2 device-info))
     (setq args (mapconcat
 		(lambda (pair)
 		  (if (member (car pair) allowed-args)