# year    value 
 1980 -9.148245 
 1981  -8.0569 
 1982 -10.3029 
 1983 -8.444068 
 1984 -9.633755 
 1985 -8.747724 
 1986 -9.046609 
 1987 -9.405209 
 1988 -8.605767 
 1989 -9.434759 
 1990 -9.913052 
 1991  -8.2008 
 1992 -10.28222 
 1993 -9.225085 
 1994 -8.615647 
 1995 -7.954495 
 1996 -8.751622 
 1997 -8.211693 
 1998 -9.162558 
 1999 -8.489529 
 2000 -8.713161 
 2001 -8.108681 
 2002 -7.966178 
 2003 -7.576478 
 2004 -8.164992 
 2005 -6.816862 
 2006 -7.999612 
 2007 -7.333787 
 2008 -7.532329 
 2009 -7.832829 
 2010 -7.126062 
 2011 -7.554861 
 2012 -7.703182 
 2013 -7.67271 
 2014 -8.075073 
 2015 -7.737015 
 2016 -7.428415 
 2017 -7.726055 
 2018 -7.31094