-# force to 'month' for $OptReportType 'average' or 'sum'
-$OptGroupBy = 'month' if ($OptReportType and $OptReportType ne 'default');
+# default to 'newsgroup' if $OptGroupBy is not set and
+# just one newsgroup is requested, but more than one month
+$OptGroupBy = 'newsgroup' if (!$OptGroupBy and $OptMonth and $OptMonth =~ /:/
+ and $OptNewsgroups and $OptNewsgroups !~ /[:*%]/);