Interpolate dates along a line
Have fun. 

with
  num_years as (select extract(year from max(cond_date)) - extract(year from min(cond_date)) +1 as years from road_condition where road_id = 100),
  generated_dates as (select add_months(trunc((select min(cond_date) from road_condition where road_id = 100), 'YYYY'), (rownum - 1) * 12) as cond_date from dual connect by level &lt;= (select years from num_years)),
  generated_data as (select rc.cond_id, rc.road_id,nvl(rc.cond_date, gd.cond_date) as cond_date, rc.condition
    from generated_dates gd left join (select * from road_condition where road_id = 100) rc on (gd.cond_date = trunc(rc.cond_date, 'YYYY'))
  ),
  data1 as (select
      last_value(cond_id ignore nulls) over (order by cond_date) as cond_id,
      cond_id as original_cond_id,
      road_id, cond_date, condition,
      last_value(condition ignore nulls) over (order by cond_date) as s1,
      lead(condition ignore nulls) over (order by cond_date) as s2
    from generated_data
  ),
  data2 as (
    select
      cond_id, road_id, original_cond_id, cond_date, condition, s1, s2,
      count(*) over (partition by cond_id) + 1 as s,
      count(*) over (partition by cond_id order by cond_date rows between unbounded preceding and current row) as s0
   from data1
  ),
  data3 as (
    select original_cond_id as cond_id, road_id, cond_date, 
     round(nvl(condition, (1-s0/s)*(s1-s2)+s2)) as condition
    from data2
  ),
  final_data as (
    select cond_id, road_id, cond_date,
      case when condition between 15 and 20 then condition end as cond_good,
      case when condition between 11 and 14 then condition end as cond_overlay,
      case when condition between 8 and 10 then condition end as cond_fdr,
     case when condition between 0 and 7 then condition end as cond_recon
    from data3
  ),
  data_for_graph as 
  (
    select 
      cond_id, road_id, cond_date, cond_good,
      case when cond_good is not null and lead(cond_overlay) over (order by cond_date) is not null then cond_good else cond_overlay end as cond_overlay,
      case when cond_overlay is not null and lead(cond_fdr) over (order by cond_date) is not null then cond_overlay else cond_fdr end as cond_fdr,
      case when cond_fdr is not null and lead(cond_recon) over (order by cond_date) is not null then cond_fdr else cond_recon end as cond_recon
    from final_data
  )
select * from data_for_graph order by cond_date;


Output:

   COND_ID    ROAD_ID COND_DATE   COND_GOOD COND_OVERLAY   COND_FDR COND_RECON
---------- ---------- ---------- ---------- ------------ ---------- ----------
         1        100 1984-11-01         18                                   
                      1985-01-01         17                                   
                      1986-01-01         17                                   
                      1987-01-01         16                                   
                      1988-01-01         16                                   
                      1989-01-01         15                                   
                      1990-01-01         15           15                      
                      1991-01-01                      14                      
                      1992-01-01                      14                      
                      1993-01-01                      13                      
                      1994-01-01                      13                      
                      1995-01-01                      12                      
                      1996-01-01                      12                      
                      1997-01-01                      12                      
                      1998-01-01                      11                      
                      1999-01-01                      11         11           
                      2000-01-01                                 10           
                      2001-01-01                                 10           
                      2002-01-01                                  9           
                      2003-01-01                                  9           
                      2004-01-01                                  8           
                      2005-01-01                                  8          8
                      2006-01-01                                             7
                      2007-01-01                                             7
                      2008-01-01                                             6
         2        100 2009-01-01                                             6
                      2010-01-01                                             5
                      2011-01-01                                             5
         3        100 2012-06-19                                             4
                      2013-01-01                                             4
                      2014-01-01                                             4
         4        100 2015-04-29                                             4


You can follow the steps by selecting from the CTEs in the last line in the order I listed them (so select * from num_years, then select * from generated_dates, then select * from generated_data, and so on).