Oracle measure number of Active Sessions

I was recently asked to produce some KPIs for our three node Oracle cluster database. One of the KPIs was to display a graph trend for total number of active sessions among all the cluster nodes sampled every minute. This can be done by querying Oracle's Automatic Workload Repository (AWR):

     select
        round(sample_time,'MI'),
        count(*) as active_sessions
     from
        dba_hist_active_sess_history
     where
        sample_time > sysdate - 7
     group by
        round(sample_time, 'MI')

You can then graph the values as in this example: