org-plot.el: add new option :transpose

* lisp/org-plot.el (org-plot/add-options-to-plist,
org-plot/add-options-to-plist): Add a new option :transpose, and a
shorter alias :trans.  Transposition is performed if the argument is yes,
y, or t.  This treats the table as a matrix and performs matrix
transposition on it.  If an hline is present, it is assumed that it is a
marks a separation from a first header row.  The first row is then
treated as the new header by inserting a hline in the transposed data.
This is quite useful for some plots, where across multiple categories,
there are a large number of data points.  Without this, the data points
would be columns and the table can spread irritatingly wide.

lisp/org-plot.el

@@ -51,19 +51,21 @@
   "Parse an OPTIONS line and set values in the property list P.
 Returns the resulting property list."
   (when options
-    (let ((op '(("type"    . :plot-type)
-		("script"  . :script)
-		("line"    . :line)
-		("set"     . :set)
-		("title"   . :title)
-		("ind"     . :ind)
-		("deps"    . :deps)
-		("with"    . :with)
-		("file"    . :file)
-		("labels"  . :labels)
-		("map"     . :map)
-		("timeind" . :timeind)
-		("timefmt" . :timefmt)))
+    (let ((op '(("type"      . :plot-type)
+		("script"    . :script)
+		("line"      . :line)
+		("set"       . :set)
+		("title"     . :title)
+		("ind"       . :ind)
+		("deps"      . :deps)
+		("with"      . :with)
+		("file"      . :file)
+		("labels"    . :labels)
+		("map"       . :map)
+		("timeind"   . :timeind)
+		("timefmt"   . :timefmt)
+		("trans"     . :transpose)
+		("transpose" . :transpose)))
 	  (multiples '("set" "line"))
 	  (regexp ":\\([\"][^\"]+?[\"]\\|[(][^)]+?[)]\\|[^ \t\n\r;,.]*\\)")
 	  (start 0))
@@ -290,8 +292,20 @@ line directly before or after the table."
 	(setf params (plist-put params (car pair) (cdr pair)))))
     ;; collect table and table information
     (let* ((data-file (make-temp-file "org-plot"))
-	   (table (org-table-collapse-header (org-table-to-lisp)))
-	   (num-cols (length (car table))))
+	   (table (let ((tbl (org-table-to-lisp)))
+		    (when (pcase (plist-get params :transpose)
+			    ('y   t)
+			    ('yes t)
+			    ('t   t))
+		      (if (memq 'hline tbl)
+			  (setq tbl (apply #'cl-mapcar #'list tbl))
+			;; When present, remove hlines as they can't (currentily) be easily transposed.
+			(setq tbl (apply #'cl-mapcar #'list
+					 (remove 'hline tbl)))
+			(push 'hline (cdr tbl))))
+		    tbl))
+	   (num-cols (length (if (eq (nth 0 table) 'hline) (nth 1 table)
+			       (nth 0 table)))))
       (run-with-idle-timer 0.1 nil #'delete-file data-file)
       (when (eq (cadr table) 'hline)
 	(setf params