In 1996, the move toward statutory restrictions on gay marriage showed a strong upward trend, which continued in 1997 and 1998. Meanwhile, in 1998, states began to constitutionally ban same-sex marriage with Alaska leading the way. That movement picked up steam in 2004, apparently in reaction to the legalization of gay marriage in Massachusetts, which was approved in 2003. In 2005 and 2006, more and more states put bans on same-sex marriage in their state constitutions. Then, in 2008 Connecticut joined Massachusetts as a green state ("states where gay marriage is or soon will be legal"). The strong movement toward gay marriage began in 2011, and the last two years have shown more and more green states.
The map for 2013 is notable for the deep divide it shows in the nation. The Northeast is green. Vast stretches of the rest of the country are yellow-brown ("states with constitutional bans on gay marriage"). A few states still have statutory bans on gay marriage and as noted above only New Mexico has not taken a stand one way or the other, but evidently some local governments in New Mexico are taking matters into their own hands (see here) by issuing marriage licenses to gay couples. So far, California is the only state that had a constitutional ban on same-sex marriage, which has since legalized such marriages.
In sum, this map provides graphic evidence of the deep divisions in our nation over same-sex marriage and demonstrates that both proponents and opponents have so far had their share of victories. Given the evident difficulty of flipping states with constitutional bans on gay marriage, however, one has to wonder whether the state-by-state campaign for equality under the law has gone about as far as it can.