drop table timings;

create table timings (seq_no number, time_of_event date, figure number);

insert into timings (seq_no, time_of_event, figure)
values (15, TO_DATE('23.5.2007 16:15', 'dd.mm.yyyy hh24:mi') , 258);

insert into timings (seq_no, time_of_event, figure)
values (16, TO_DATE('23.5.2007 16:30', 'dd.mm.yyyy hh24:mi') , 339);

insert into timings (seq_no, time_of_event, figure)
values (17, TO_DATE('23.5.2007 16:45', 'dd.mm.yyyy hh24:mi') , null);

insert into timings (seq_no, time_of_event, figure)
values (18, TO_DATE('23.5.2007 17:00', 'dd.mm.yyyy hh24:mi') , 356);

insert into timings (seq_no, time_of_event, figure)
values (19, TO_DATE('23.5.2007 17:15', 'dd.mm.yyyy hh24:mi') , 373);

insert into timings (seq_no, time_of_event, figure)
values (20, TO_DATE('23.5.2007 17:30', 'dd.mm.yyyy hh24:mi') , 355);

insert into timings (seq_no, time_of_event, figure)
values (21, TO_DATE('23.5.2007 17:45', 'dd.mm.yyyy hh24:mi') , 363);

insert into timings (seq_no, time_of_event, figure)
values (22, TO_DATE('23.5.2007 18:00', 'dd.mm.yyyy hh24:mi') , 355);

insert into timings (seq_no, time_of_event, figure)
values (23, TO_DATE('23.5.2007 18:15', 'dd.mm.yyyy hh24:mi') ,null);

insert into timings (seq_no, time_of_event, figure)
values (24, TO_DATE('23.5.2007 18:30', 'dd.mm.yyyy hh24:mi') ,null);

insert into timings (seq_no, time_of_event, figure)
values (25, TO_DATE('23.5.2007 19:00', 'dd.mm.yyyy hh24:mi') , 292);

insert into timings (seq_no, time_of_event, figure)
values (26, TO_DATE('23.5.2007 19:15', 'dd.mm.yyyy hh24:mi') , 295);


select seq_no, time_of_event, next_non_null_fig,
prev_non_null_fig + (((tot_rows-null_pos)/tot_rows)*(next_non_null_fig - prev_non_null_fig)) as computed 
from
(select 
t.seq_no, t.figure, t.time_of_event, pt.time_of_event as prev_non_null_time, nt.time_of_event as next_non_null_time,
pt.figure as prev_non_null_fig, nt.figure as next_non_null_fig,
(select count(*)-1 from timings t1 where t1.seq_no between t.seq_no and t.next_non_null_seq_no) as null_pos,
(select count(*)-1 from timings t1 where t1.seq_no between t.prev_non_null_seq_no and t.next_non_null_seq_no) as tot_rows
from
(select seq_no, figure, 
time_of_event,
(select max(t2.seq_no) from timings t2 where t2.time_of_event < t1.time_of_event and t2.figure is not null) as prev_non_null_seq_no ,
(select min(t2.seq_no) from timings t2 where t2.time_of_event > t1.time_of_event and t2.figure is not null) as next_non_null_seq_no
from timings t1) t, timings nt, timings pt
where nt.seq_no   = t.next_non_null_seq_no 
and   pt.seq_no   = t.prev_non_null_seq_no
and   t.figure is null)
order by seq_no

drop table temp_table;

create table temp_table (starttime date,data_column number);

insert into temp_table values ( to_date('23.5.2007 16:15','dd.mm.yyyy hh24:mi') , 258);
insert into temp_table values ( to_date('23.5.2007 16:30','dd.mm.yyyy hh24:mi') , 339);
insert into temp_table values ( to_date('23.5.2007 16:45','dd.mm.yyyy hh24:mi') , null);
insert into temp_table values ( to_date('23.5.2007 17:00','dd.mm.yyyy hh24:mi') , 356);
insert into temp_table values ( to_date('23.5.2007 17:15','dd.mm.yyyy hh24:mi') , 373);
insert into temp_table values ( to_date('23.5.2007 17:30','dd.mm.yyyy hh24:mi') , 355);
insert into temp_table values ( to_date('23.5.2007 17:45','dd.mm.yyyy hh24:mi') , 363);
insert into temp_table values ( to_date('23.5.2007 18:00','dd.mm.yyyy hh24:mi') , 355);
insert into temp_table values ( to_date('23.5.2007 18:15','dd.mm.yyyy hh24:mi') , null);
insert into temp_table values ( to_date('23.5.2007 18:30','dd.mm.yyyy hh24:mi') , null);
insert into temp_table values ( to_date('23.5.2007 19:00','dd.mm.yyyy hh24:mi') , 292);
insert into temp_table values ( to_date('23.5.2007 19:15','dd.mm.yyyy hh24:mi') , 295);

select starttime
      ,data_column    orig_data_col
      ,nvl(data_column ,lnn + (nnn-lnn)*(null_numerator/null_denominator)) data_column
from  (      
select starttime
      ,data_column
      ,rnum
      ,last_non_null,next_non_null
      ,first_value(data_column) over (partition by last_non_null order by data_column nulls last) lnn
      ,first_value(data_column) over (partition by next_non_null order by data_column nulls last) nnn      
      ,count(1) over (partition by last_non_null order by starttime rows between unbounded preceding and current row)-1 null_numerator
      ,sum(1) over (partition by next_non_null+last_non_null)+1 null_denominator
from (
select starttime
      ,data_column
      ,row_number() over (order by starttime) rnum
      ,row_number() over (order by starttime) -count(nvl2(data_column,null,'A')) over (order by starttime) last_non_null
      ,row_number() over (order by starttime)  +count(nvl2(data_column,null,'A')) over (order by starttime desc) next_non_null
from   temp_table)
)

STARTTIME      ORIG_DATA_COL DATA_COLUMN
-------------- ------------- -----------
23/05/07 16:15           258         258
23/05/07 16:30           339         339
23/05/07 16:45                     347.5
23/05/07 17:00           356         356
23/05/07 17:15           373         373
23/05/07 17:30           355         355
23/05/07 17:45           363         363
23/05/07 18:00           355         355
23/05/07 18:15                       334
23/05/07 18:30                       313
23/05/07 19:00           292         292
23/05/07 19:15           295         295

select starttime
      ,data_column
      ,rnum
      ,rnum -count(nvl2(data_column,null,'A')) over (order by starttime) last_non_null
      ,rnum +count(nvl2(data_column,null,'A')) over (order by starttime desc) next_non_null
from   (
select starttime
      ,data_column
      ,row_number() over (order by starttime) rnum
from  temp_table)