FYI -- your module doesn't work with PostgreSQL.

There are several SQL orders that are not compatible with PostgreSQL.

query: SELECT COUNT(*) as count, r.* FROM users_roles ur
     LEFT JOIN users u ON ur.uid = u.uid
     LEFT JOIN role r ON r.rid = ur.rid
     GROUP BY r.rid ORDER BY r.name
  in /usr/clients/www/drupal/sites/all/modules/chart/contrib/system_charts/system_charts.module on line 269.

The error is explained below:

pg_query() [function.pg-query]: Query failed: ERROR: column "r.name" must appear in the GROUP BY clause or be used in an aggregate function in /usr/clients/www/drupal/includes/database.pgsql.inc on line 187.

So you need to add r.name in the GROUP BY as in:

  GROUP BY r.rid, r.name

It makes no difference to MySQL which does not follow the SQL references to the letter (and thus is quite bogus.)

Thank you.
Alexis