I think it's most important that you feel both comfortable and confident.
I have been to three graduations, and there does not seem to be a dress code. Some mothers wore smart suits, others 'garden party' day dresses and many wore nice trouser suits.
Don't forget it is likely to be warm.
As you will be photographed with your daughter, I suggest you avoid black - you will merge in to her graduation gown. Think about your heels too, especially if you are likely to be standing on lawned areas for any length of time.