That is from this paper by Lei Zhang.
Some data (p. 11)
Annual college admission growth averages 4.7% between 1995 and 1998; it grows by more than 40% in both 1999 and 2000 and by about 20% in the next four years before it starts to taper off in 2006 (Figure 4, top Panel). Nationwide college enrolment doubles between 1998 and 2001 and doubles again between 2001 and 2005 (Figure 4, bottom Panel). Statistics are reported in Appendix Table 1. The gross college enrolment rate relative to the 18-22 population increases from 9.8% in 1998 to 24.2% in 2009, the last year for which data are available.