Highly visible Progress
Some things of note. When the Supreme Court handed down Loving vs. Virginia they were very much "forcing mixed marriage down our throats" (in modern conservative speak) since mixed marriage was a lot less popular circa 1968 than same sex marriage is today. And mixed marriage was surely legal in a lot more states in 1968 than same sex marriage is today. Looked at that way, we are way overdue on marriage equality. That said, mixed race marriage was an easier issue because it didn't really involve the nature of the institution of marriage, but rather formal segregation in the south, which had been taking judicial hits for a generation before Loving.
Also, the smoothness of the line is such that it looks like a generational turnover. The only seeming big move coincides with nothing I can think of except the impeachment of Bill Clinton. Perhaps that generated a distaste for telling anyone what to do behind closed doors. But it could also just be statistical noise.
The polling set, being approval rather than a question about legality, is as much about total cultural integration as white racism since cultural opposition to biracial marriage had long been a factor in both white and black culture. (Heck, it was just a few weeks ago that a black man was fired from ESPN for citing RG3's white fiance as an indication that RG3 is somehow a fraud.)
But whatever is being measured, it's a remarkable progress story.
And boy was Obama's birth controversial when he was born! (1961, I think)