# year    value 
 1980 21.04315 
 1981 20.84852 
 1982 21.03777 
 1983 21.08147 
 1984 20.90046 
 1985 20.91055 
 1986 21.09069 
 1987 21.40251 
 1988 21.26218 
 1989  20.8284 
 1990 21.05001 
 1991 21.31888 
 1992 21.18137 
 1993 21.29476 
 1994  21.0947 
 1995 21.57866 
 1996 21.36279 
 1997 21.41629 
 1998 21.85976 
 1999 21.35156 
 2000 21.14712 
 2001 21.41688 
 2002 21.51436 
 2003 21.51679 
 2004 21.31856 
 2005 21.79444 
 2006 21.43286 
 2007   21.133 
 2008 21.37272 
 2009 21.61915 
 2010 21.48197 
 2011 21.37116 
 2012 21.31626 
 2013 21.61802 
 2014 21.56061 
 2015 21.89808 
 2016 21.75206 
 2017  21.9252 
 2018 21.40613