Category Tree: Reduce number of categories

Don't use variable to set category, plugin gallery build script needs the pure string..

plugins/mysql/mysql-table-size

@@ -30,7 +30,7 @@ EOT;
 if($argc > 1 && $argv[1] == 'config') {
   $text = <<<EOT
 graph_title $dbname Table Data Size
-graph_category mysql
+graph_category db
 graph_vlabel Size
 graph_args --base 1024 -l 0
 $text