Merge pull request #852 from ncb000gt/master

Return null from getSelection if there's no selection.

jquery.flot.selection.js

@@ -147,6 +147,8 @@ The plugin allso adds the following methods to the plot object:
             if (!selectionIsSane())
                 return null;
             
+            if (!selection.show) return null;
+
             var r = {}, c1 = selection.first, c2 = selection.second;
             $.each(plot.getAxes(), function (name, axis) {
                 if (axis.used) {