SELECT TRUNC (r.react_date, 'HH24') AS hour,
COUNT (*) AS cnt
FROM REACT r
WHERE r.react_date BETWEEN :P1_DAT1 AND :P1_DAT2
GROUP BY TRUNC (r.react_date, 'HH24')
ORDER BY TRUNC (r.react_date, 'HH24') ASC

и еще кнопку "применить" submit page