# year    value 
 1979 -10.1918 
 1980 -9.263646 
 1981 -8.335314 
 1982 -9.951598 
 1983 -9.245265 
 1984 -9.430361 
 1985 -9.486553 
 1986 -9.564841 
 1987 -9.850359 
 1988 -8.824589 
 1989 -9.123734 
 1990 -9.240273 
 1991 -8.963181 
 1992 -9.812477 
 1993 -8.99944 
 1994 -9.284482 
 1995 -8.084039 
 1996 -8.694182 
 1997 -8.855554 
 1998 -8.990555 
 1999 -9.198427 
 2000 -8.677257 
 2001 -8.611009 
 2002  -8.5998 
 2003 -7.953435 
 2004 -9.151237 
 2005 -7.574014 
 2006 -8.099272 
 2007 -7.737103 
 2008 -8.06422 
 2009 -8.629086 
 2010 -7.515945 
 2011 -7.54582 
 2012 -7.675966 
 2013 -8.264775 
 2014 -8.030316 
 2015 -7.896074 
 2016 -6.782591 
 2017 -7.286455 
 2018 -7.830624