[FIX] correctly compute mouse click coords

in graph view, when the user click on the pivot table.  It was
calculated with pageX, pageY and positioned with 'absolute'. However,
the main view is now also positionod, so the dropdown menu was not
where it should be.

addons/web_graph/static/src/js/graph_widget.js

@@ -320,8 +320,8 @@ openerp.web_graph.Graph = openerp.web.Widget.extend({
         this.dropdown = $(QWeb.render('field_selection', {fields:fields, header_id:id}));
         $(event.target).after(this.dropdown);
         this.dropdown.css({position:'absolute',
-                           left:event.pageX,
-                           top:event.pageY});
+                           left:event.originalEvent.layerX,
+                           top:event.originalEvent.layerY});
         this.$('.field-selection').next('.dropdown-menu').toggle();