# year    value 
 1979 23.01246 
 1980 23.52815 
 1981 23.26391 
 1982 22.98173 
 1983 23.69729 
 1984 23.11422 
 1985 23.15109 
 1986 23.43431 
 1987 23.57716 
 1988 23.51995 
 1989 22.96137 
 1990 23.56109 
 1991 23.42429 
 1992 23.66478 
 1993 23.53227 
 1994 23.35802 
 1995 23.27106 
 1996 23.32537 
 1997 23.03007 
 1998  24.1642 
 1999 23.22717 
 2000 22.90362 
 2001  23.1692 
 2002 23.84576 
 2003 23.59111 
 2004 23.41209 
 2005 23.94682 
 2006 23.07991 
 2007 23.70374 
 2008 23.10644 
 2009 23.52667 
 2010 23.95188 
 2011 23.12384 
 2012 23.35209 
 2013  23.7742 
 2014 23.68225 
 2015 23.70687 
 2016 24.34304 
 2017 23.84511 
 2018 23.82809