# year    value 
 1901 20.34542 
 1902 20.28195 
 1903 20.50473 
 1904 19.88619 
 1905 20.06603 
 1906 20.26565 
 1907  20.0186 
 1908 20.24396 
 1909 20.09744 
 1910 19.94011 
 1911 19.98857 
 1912 20.70323 
 1913 20.21845 
 1914 20.33436 
 1915 20.59407 
 1916 20.24798 
 1917 19.88566 
 1918  19.7119 
 1919 20.37451 
 1920 20.37107 
 1921 20.18221 
 1922 20.46734 
 1923 20.38502 
 1924 20.48922 
 1925 20.18997 
 1926 20.77075 
 1927  20.5476 
 1928  20.5805 
 1929 20.46197 
 1930 20.39988 
 1931   20.584 
 1932 20.37127 
 1933  20.2484 
 1934 20.09502 
 1935 20.25064 
 1936 20.36565 
 1937 20.43195 
 1938 20.36722 
 1939 20.45312 
 1940 20.24092 
 1941 20.74893 
 1942 20.87052 
 1943   20.392 
 1944 20.47862 
 1945 20.71401 
 1946   20.605 
 1947    20.63 
 1948 20.35425 
 1949 20.40826 
 1950 20.16422 
 1951 19.96035 
 1952  20.4374 
 1953 20.44478 
 1954 20.37581 
 1955 20.28442 
 1956 19.98053 
 1957 20.03805 
 1958 20.74416 
 1959 20.46106 
 1960 20.32284 
 1961  20.5608 
 1962 20.25864 
 1963 20.24629 
 1964 20.53777 
 1965 20.13477 
 1966 20.54808 
 1967 20.47284 
 1968 20.18629 
 1969 20.72923 
 1970 20.59154 
 1971 20.16129 
 1972 20.24927 
 1973 21.04546 
 1974 20.00217 
 1975 20.04173 
 1976 20.00819 
 1977 20.57777 
 1978 20.37598 
 1979 20.20909 
 1980 20.50609 
 1981 20.47308 
 1982 20.50404 
 1983 21.02487 
 1984 20.53943 
 1985 20.32916 
 1986 20.45882 
 1987 20.85476 
 1988 20.94927 
 1989 20.44383 
 1990 20.56019 
 1991   20.611 
 1992  20.6344 
 1993 20.50456 
 1994 20.67772 
 1995 20.93619 
 1996 20.57387 
 1997 20.57283 
 1998 21.26172 
 1999 20.71137 
 2000 20.55409 
 2001 20.73119 
 2002 20.57542 
 2003 21.05111 
 2004 20.91406 
 2005 21.04365 
 2006 20.81453 
 2007 20.87505 
 2008 20.48584 
 2009 20.75188 
 2010 21.22871 
 2011 20.64337 
 2012 20.63997 
 2013 20.97164 
 2014 20.88386 
 2015  20.8478 
 2016 21.53511 
 2017 21.23385 
 2018 20.76929