This is the World Wildlife Fund-Mexico graph (I had to manually add the 2008 data to it): http://i636.photobucket.com/albums/uu87/4ALC/wwf.jpg
It has slightly different figures than the Journey North graph and a decline is less evident. A good count this winter (6 hectares or more) could cast doubt on whether or not there really has been a decline, especially considering the Cape May, New Jersey data shows no decline since counts began in 1992.
Meanwhile, the California overwintering population is down a clear cut whopping 75% from the pre-1992 years because in the 1980's and 90's conservationists focused all their efforts on overwintering site protection and ignored milkweed protection.
There's big money to be made in California overwintering site protection (e.g. 5 figure grove management consulting jobs for Monarch biologists) but no money to be made in western milkweed patch protection so that may partially explain the past and ongoing lack of interest in western milkweed patch protection. The net result is a paradox; i.e. on the one hand the USA demands Mexico do this and that to protect their forests to preserve the eastern monarch migration while we let our own western migration go to pot.