YABU. This thread actually makes me angry.
I'm about to finish my PhD and I'm not going to the ceremony. Why exactly is the ceremony so important to you? You seem to assume that of course it is but clearly it's not for everybody. You should be proud of what you've done but you don't need to walk across the stage to be proud or to celebrate. Have a big family party the following weekend, that will be more fun anyway.
If you go back to your DD now and tell her how hurt you are, etc., then yes I think that's emotional blackmail. She has made her choice, now please get a grip and try not to be so upset about it.
And as for saying a PhD is more impressive than a marriage -- wow, are you for real? Do you seriously not know how many idiots out there have PhDs?
I am proud of what I've done but I don't think it's that impressive, not to the extent that I expect other people to celebrate my achievement or whatever. It's not like I've won the Nobel Prize or something.
So please, be proud of yourself, have a great time at the ceremony, and find some other way to celebrate with your DD, maybe something just the two of you since you feel like she was such a big part of it.