# year    value 
 1979 26.58019 
 1980 26.57405 
 1981 26.61766 
 1982 26.46053 
 1983 26.79457 
 1984 26.53238 
 1985 26.54598 
 1986 26.47028 
 1987 26.79399 
 1988 26.81633 
 1989 26.34599 
 1990 26.65674 
 1991 26.84574 
 1992 26.30581 
 1993 26.32689 
 1994 26.81244 
 1995 27.07842 
 1996 26.47686 
 1997 27.04244 
 1998 27.23005 
 1999 26.96786 
 2000 27.09665 
 2001 26.94141 
 2002 27.32791 
 2003 26.67875 
 2004 26.79918 
 2005 27.43981 
 2006 27.20612 
 2007 27.37565 
 2008 27.04155 
 2009 27.24701 
 2010 27.54425 
 2011  27.3276 
 2012 27.35159 
 2013 27.01273 
 2014 27.29398 
 2015 27.21951 
 2016 27.49853 
 2017 27.47372 
 2018  27.6042