# year    value 
 1980 21.65697 
 1981 21.64404 
 1982  20.8484 
 1983 21.32217 
 1984 21.33167 
 1985 21.43448 
 1986 21.44787 
 1987 21.09832 
 1988 22.25361 
 1989 21.27816 
 1990 21.62353 
 1991 21.27421 
 1992 20.37978 
 1993 20.02071 
 1994 22.28743 
 1995 21.13295 
 1996 21.86534 
 1997 21.13997 
 1998 21.62644 
 1999 21.01198 
 2000 21.99366 
 2001 22.08584 
 2002 22.27857 
 2003 22.52371 
 2004  20.9581 
 2005 21.78103 
 2006  22.8348 
 2007 22.50821 
 2008 21.79257 
 2009 21.28112 
 2010 21.84163 
 2011 22.49981 
 2012 22.97766 
 2013 22.39875 
 2014 21.65442 
 2015 22.49372 
 2016  22.6122 
 2017 22.68765 
 2018 22.78031