--- a/scripts/plot/__add_datasource__.m
+++ b/scripts/plot/__add_datasource__.m
@@ -32,17 +32,18 @@
     addproperty (strcat (data{i}, "datasource"), h, "string", "");
   endfor
 
-  i = 1;
+  i = 0;
   newargs = {};
   while (i < numel (varargin))
-    arg = varargin{i++};
-    if (ischar (arg) && length (arg > 1) && strcmpi (arg(2:end), "datasource"))
+    arg = varargin{++i};
+    if (i != numel(varargin) && ischar (arg) &&
+    length (arg > 1) && strcmpi (arg(2:end), "datasource"))
       arg = tolower (arg);
-      val = varargin{i++};
-      if (ischar (val)) 
-	set (h, arg, val);
+      val = varargin{++i};
+      if (ischar (val))
+    set (h, arg, val);
       else
-	error ("%s: expecting data source to be a string", fcn);
+    error ("%s: expecting data source to be a string", fcn);
       endif
     else
       newargs{end + 1} = arg;