| Year | Jan | Feb | March | April | May | June | July | Aug | Sept | Oct | Nov | Dec |
| 1983 | 31.65 | 29.38 | 28.35 | 29.6 | 29.76 | 30.13 | 30.66 | 31.04 | 30.51 | 29.93 | 29.31 | 28.78 |
| 1984 | 29.32 | 29.7 | 30.05 | 30.11 | 29.93 | 29.33 | 28.1 | 28.4 | 28.48 | 28.15 | 27.8 | 27.31 |
| 1985 | 27.09 | 27.89 | 28.04 | 27.98 | 26.85 | 26.66 | 26.97 | 27.79 | 28.09 | 28.87 | 29.89 | 27.25 |
| 1986 | 22.38 | 17.4 | 14.21 | 12.5 | 14.41 | 12.09 | 9.82 | 13.91 | 14.11 | 13.6 | 14.49 | 16.12 |
| 1987 | 18.4 | 17.43 | 18.2 | 18.25 | 18.84 | 19.01 | 20.1 | 19.1 | 18.55 | 19.03 | 18.05 | 17.28 |
| 1988 | 16.85 | 15.8 | 15.02 | 16.76 | 16.57 | 15.38 | 15.39 | 14.9 | 13.34 | 12.59 | 13.33 | 15.59 |
| 1989 | 17.27 | 17.23 | 19.18 | 20.49 | 18.96 | 17.85 | 18.14 | 16.87 | 17.93 | 19.26 | 19 | 20.28 |
| 1990 | 21.64 | 20.04 | 18.73 | 17.1 | 16.75 | 15.46 | 17.86 | 28.6 | 35.31 | 36.95 | 33.55 | 28.85 |
| 1991 | 24.55 | 20.25 | 19.36 | 19.41 | 19.39 | 18.53 | 19.81 | 20.18 | 21.03 | 22.81 | 21.74 | 18.91 |
| 1992 | 18.6 | 18.63 | 18.22 | 19.75 | 20.55 | 21.76 | 20.94 | 20.46 | 20.86 | 20.95 | 19.91 | 18.83 |
| 1993 | 17.8 | 19.13 | 19.42 | 19.24 | 19.01 | 18.25 | 17.51 | 17.22 | 16.44 | 17.08 | 15.66 | 13.96 |
| 1994 | 14.74 | 14.5 | 14.4 | 15.55 | 16.72 | 17.21 | 17.85 | 16.98 | 16.01 | 16.89 | 17.58 | 15.94 |
| 1995 | 16.92 | 17.54 | 17.24 | 18.84 | 18.71 | 17.58 | 15.95 | 16.25 | 17.11 | 16.56 | 17.19 | 18.44 |
| 1996 | 18.55 | 18.64 | 20.64 | 21.43 | 19.58 | 18.73 | 20.04 | 21.15 | 22.95 | 24.74 | 23.11 | 24.53 |
| 1997 | 24.04 | 21.65 | 19.39 | 17.82 | 19.6 | 17.95 | 18.95 | 19.04 | 18.89 | 19.98 | 19.36 | 17.34 |
| 1998 | 15.25 | 14.11 | 13.14 | 13.51 | 14.46 | 11.89 | 12.01 | 12.14 | 13.59 | 12.66 | 11.15 | 9.96 |
| 1999 | 11.33 | 10.24 | 12.56 | 15.44 | 15.45 | 15.86 | 19.28 | 20.44 | 22.9 | 22.3 | 24.8 | 25.86 |
| 2000 | 25.41 | 28.36 | 27.54 | 22.91 | 27.87 | 29.86 | 28.75 | 29.06 | 32.65 | 30.67 | 32.86 | 25.47 |
| 2001 | 25.43 | 27.4 | 24.35 | 25.43 | 28.51 | 28.06 | 24.81 | 25.41 | 25.98 | 20.6 | 18.92 | 18.78 |
| 2002 | 19.65 | 20.3 | 23.76 | 25.79 | 25.1 | 23.98 | 25.93 | 26.94 | 28.46 | 27.9 | 24.07 | 29.27 |
| 2003 | 30.78 | 32.33 | 30.83 | 25.27 | 25.78 | 27.46 | 28.39 | 29.79 | 27.47 | 29.59 | 28.93 | 29.64 |
| 2004 | 30.94 | 30.47 | 33.34 | 33.74 | 37.87 | 35.6 | 38.08 | 42.55 | 43.56 | 49.91 | 43.6 | 39.08 |
| 2005 | 44.01 | 45.43 | 53.15 | 53.18 | 50.23 | 55.93 | 58.4 | 65.49 | 65.6 | 60.74 | 57.18 | 57.91 |
| 2006 | 64.04 | 62.12 | 63.8 | 71.8 | 71.75 | 70.22 | 75.49 | 75.29 | 63.87 | 58.75 | 60.32 | 64.28 |
| 2007 | 56.18 | 59.58 | 64.6 | 70.01 | 70.03 | 74.45 | 79.21 | 73.34 | 79.87 | 85.6 | 95.32 | 93.55 |
| 2008 | 94.85 | 96.98 | 106.68 | 112.52 | 126.55 | 136.44 | 137.64 | 116.93 | 100.48 | 74.57 | 56.11 | 43.1 |
| 2009 | 45.44 | 45.07 | 49.7 | 52.24 | 57.87 | 69.55 | 66.31 | 73.84 | 68.74 | 74.41 | 77.96 | 75.68 |
| 2010 | 77.39 | 75.04 | 80.4 | 86.14 | 76.87 | 76 | 77.04 | 78.82 | 79.65 | 84.35 | 86.83 | 93.08 |
| 2011 | 98.1 | 105.66 | 116.75 | 127.12 | 118.88 | 117.27 | 119.69 | 112.41 | 115.63 | 113.09 | 114.21 | 110.71 |
| 2012 | 113.08 | 122.36 | 127.98 | 122.36 | 112.87 | 97.19 | 104.24 | 114.63 | 114.06 | 113.31 | 110.91 | 111.19 |
| 2013 | 115.41 | 118.69 | 110.57 | 105.17 | 105.83 | 106.12 | 110.21 | 113.62 | 114.3 | 112.44 | 111.47 | 113.11 |