# year    value 
 1980 23.30768 
 1981  23.0904 
 1982 23.16178 
 1983 23.50453 
 1984 23.10991 
 1985 23.20177 
 1986 23.28917 
 1987 23.57828 
 1988 23.45948 
 1989 23.10539 
 1990 23.40453 
 1991 23.40532 
 1992 23.37529 
 1993 23.38954 
 1994 23.40947 
 1995 23.53012 
 1996 23.37146 
 1997 23.51535 
 1998 23.86832 
 1999 23.34989 
 2000 23.23725 
 2001 23.42206 
 2002 23.71126 
 2003 23.69871 
 2004 23.59227 
 2005 23.85622 
 2006 23.54426 
 2007 23.54071 
 2008 23.42887 
 2009 23.67129 
 2010 23.68294 
 2011 23.36445 
 2012 23.54401 
 2013 23.70547 
 2014 23.75252 
 2015 23.92263 
 2016 23.92691 
 2017 23.84805 
 2018 23.71343