Prueba algo como esto (ejemplo simplificado):
with
months_int as
(select trunc(min(inc_date), 'MM') min_month, trunc(max(inc_date), 'MM') max_month
from data),
months as
(
select add_months(min_month, level-1) mnth_date
from months_int
connect by add_months(min_month, level-1)<= max_month
)
select mnth_date, sum(cnt)
from data right outer join months on trunc(inc_date, 'MM') = mnth_date
group by mnth_date
order by mnth_date
Aquí hay un ejemplo de sqlfiddle
