# year    value 
 1980 20.01321 
 1981 19.92276 
 1982 20.48256 
 1983 20.42797 
 1984 20.45496 
 1985 20.35113 
 1986 20.30183 
 1987 20.89032 
 1988 20.66892 
 1989 20.28957 
 1990 20.57537 
 1991 20.31291 
 1992 20.09378 
 1993 20.26275 
 1994 20.89079 
 1995 20.78737 
 1996 20.32401 
 1997  20.9232 
 1998 20.35717 
 1999  20.3517 
 2000 20.31213 
 2001 20.37912 
 2002 20.80371 
 2003 20.78954 
 2004 20.87463 
 2005 20.12436 
 2006 20.61043 
 2007 20.48034 
 2008 20.71685 
 2009 20.85237 
 2010 20.45585 
 2011 20.82566 
 2012 21.12786 
 2013 20.47965 
 2014 21.29338 
 2015 20.89264 
 2016 20.65564 
 2017 20.68529 
 2018 20.87729