Oracle system statistic average by hour

select      to_char (begin_interval_time, 'hh24' ) snap_time, avg(value) avg_value
from        dba_hist_sysstat
natural join
            dba_hist_snapshot
where
            stat_name = '&stat_name'
group by    to_char(begin_interval_time, 'hh24')
order by    to_char (begin_interval_time, 'hh24');

Query uses dba_hist_sysstat view to show average values by hour or the day.