create table graph_lines (series number, thedate date, thevalue number);
insert into graph_lines
select 1 series
  ,add_months(trunc(sysdate,'yy'),rownum-1) thedate
  ,log(2,rownum) thevalue
from dual
connect by level <= 12
union all
select 2
  ,add_months(trunc(sysdate,'yy'),rownum-1)
  ,case when mod(rownum,3) =0 or rownum =2 then log(3,rownum) end
from dual
connect by level <= 12;
delete graph_lines where thevalue is null;