# year    value 
 1979 20.73209 
 1980 21.12541 
 1981 20.65222 
 1982 20.50199 
 1983 20.59405 
 1984 21.00996 
 1985  20.6792 
 1986 20.64196 
 1987 20.75704 
 1988 21.19013 
 1989 20.56364 
 1990 20.65894 
 1991 21.05796 
 1992 20.62905 
 1993 20.59749 
 1994 21.09244 
 1995  21.0469 
 1996 20.78144 
 1997 20.45382 
 1998 21.65857 
 1999  21.3577 
 2000 21.15078 
 2001 21.21485 
 2002 21.42499 
 2003 21.38504 
 2004 21.38588 
 2005 21.25765 
 2006 21.34641 
 2007 21.40235 
 2008 21.35262 
 2009 21.48239 
 2010 21.87812 
 2011 21.19952 
 2012 21.37097 
 2013 21.60978 
 2014 21.47848 
 2015 21.51029 
 2016 22.06645 
 2017 21.67216 
 2018 21.71916