# year    value 
 1980 20.50609 
 1981 20.47308 
 1982 20.50404 
 1983 21.02487 
 1984 20.53943 
 1985 20.32916 
 1986 20.45882 
 1987 20.85476 
 1988 20.94927 
 1989 20.44383 
 1990 20.56019 
 1991   20.611 
 1992  20.6344 
 1993 20.50456 
 1994 20.67772 
 1995 20.93619 
 1996 20.57387 
 1997 20.57283 
 1998 21.26172 
 1999 20.71137 
 2000 20.55409 
 2001 20.73119 
 2002 20.57542 
 2003 21.05111 
 2004 20.91406 
 2005 21.04365 
 2006 20.81453 
 2007 20.87505 
 2008 20.48584 
 2009 20.75188 
 2010 21.22871 
 2011 20.64337 
 2012 20.63997 
 2013 20.97164 
 2014 20.88386 
 2015  20.8478 
 2016 21.53511 
 2017 21.23385 
 2018 20.76929