How do I interpolate months in PLSQL
It is a calendar you need. How to get it? Using a row generator technique.
As I don't have your table, I'll use Scott's EMP and count employees hired in every month. Calendar I'm going to create will contain months between MIN and MAX hiredate (you can create any dates you want, of course).
EMP table's contents:
SQL&gt; select empno, ename, hiredate
  2  from emp
  3  order by hiredate;

     EMPNO ENAME      HIREDATE
---------- ---------- ----------
      7369 SMITH      17.12.1980
      7499 ALLEN      20.02.1981
      7521 WARD       22.02.1981
      7566 JONES      02.04.1981
      7698 BLAKE      01.05.1981
      7782 CLARK      09.06.1981
      7844 TURNER     08.09.1981
      7654 MARTIN     28.09.1981
      7839 KING       17.11.1981
      7900 JAMES      03.12.1981
      7902 FORD       03.12.1981
      7934 MILLER     23.01.1982
      7788 SCOTT      09.12.1982
      7876 ADAMS      12.01.1983

14 rows selected.

SQL&gt;

Query that does the job:
SQL&gt; with
  2  -- MIN and MAX hiredate
  3  minimax as
  4    (select min(trunc(hiredate, 'mm')) mindat,
  5            max(trunc(hiredate, 'mm')) maxdat
  6     from emp
  7    ),
  8  calendar (datum) as
  9  -- calendar, based on MIN and MAX hiredate
 10    (select add_months(mindat, level)
 11     from minimax
 12     connect by level &lt;= months_between(maxdat, mindat)
 13    )
 14  -- the final result
 15  select
 16    to_char(c.datum, 'yyyy-Month', 'nls_date_language = english') datum,
 17    count(e.empno) cnt
 18  from calendar c left join emp e on trunc(e.hiredate, 'mm') = trunc(c.datum, 'mm')
 19  group by c.datum
 20  order by c.datum;

DATUM                                            CNT
----------------------------------------- ----------
1981-January                                       0
1981-February                                      2
1981-March                                         0
1981-April                                         1
1981-May                                           1
1981-June                                          1
1981-July                                          0
1981-August                                        0
1981-September                                     2
1981-October                                       0
1981-November                                      1
1981-December                                      2
1982-January                                       1
1982-February                                      0
1982-March                                         0
1982-April                                         0
1982-May                                           0
1982-June                                          0
1982-July                                          0
1982-August                                        0
1982-September                                     0
1982-October                                       0
1982-November                                      0
1982-December                                      1
1983-January                                       1

25 rows selected.

SQL&gt;