| Year | Unit (M) | Growth (pct.) | Year | Unit (M) | Growth (pct.) |
| 1997 | 80.6 | 13.7 | 2007 | 271.2 | 13.3 |
| 1998 | 92.9 | 15.3 | 2008 | 302.2 | 11.4 |
| 1999 | 113.5 | 22.2 | 2009 | 305.9 | 1.2 |
| 2000 | 134.7 | 18.7 | 2010 | 351 | 14.7 |
| 2001 | 128.1 | −4.9 | 2011 | 352.8 | 0.5 |
| 2002 | 132.4 | 3.4 | 2012 | 352.7 | 0.0 |
| 2003 | 168.9 | 27.6 | 2013 | 316 | −10.4 |
| 2004 | 189.0 | 11.9 | 2014 | 315.9 | −0.2 |
| 2005 | 218.5 | 15.6 | 2015 | 287.7 | −8.9 |
| 2006 | 239.4 | 9.6 | 2016 | 269.7 | −6.3 |