org-plot: Use labels with 2d histogram plot

* lisp/org-plot.el (org-plot/preset-plot-types): When the :with
parameter is histograms, use text indices as the x-values.

lisp/org-plot.el

@@ -309,7 +309,8 @@ the argument, and must return a string to be used."
 		 (with (if (eq type 'grid) 'pm3d (plist-get params :with)))
 		 (ind (plist-get params :ind))
 		 (deps (if (plist-member params :deps) (plist-get params :deps)))
-		 (text-ind (plist-get params :textind))
+		 (text-ind (or (plist-get params :textind)
+                               (eq (plist-get params :with) 'histograms)))
 		 (col-labels (plist-get params :labels))
 		 res)
 	    (dotimes (col num-cols res)