Year	Measure
1800	538.4
1820	538.4
1913	993.3
1970	3949
1971	4138
1972	4334
1973	4598
1974	4676
1975	4382
1976	3992
1977	4320
1978	4535
1979	5106
1980	5532
1981	5882
1982	5973
1983	6453
1984	7024
1985	7694
1986	8514
1987	9474
1988	10430
1989	11210
1990	11570
1991	11750
1992	11680
1993	12040
1994	12480
1995	11650
1996	12100
1997	12390
1998	12610
1999	12830
2000	12740
2001	12770
2002	12870
2003	13350
2004	14110
2005	14580
2006	16220
2007	17160
2008	17230
2009	15220
2010	13920
2011	13510
2012	13720
2013	13950
2014	14400
2015	14880
2016	15400
2017	15940
2018	16500