To dance on somebody's grave is to rejoice that they're dead (or figuratively that they have been completely defeated in some way, or suffered some great misfortune, they are in some way "dead and gone").
You heard the news that Mr Badman has been fired? I'm sure I won't be the only one dancing on his grave (lots of people will be happy that he is gone).
"we all dance on our graves" is not an expression I have heard before, what it means would depend on the context.
Two men, lecturers at a university, who are close friends, are having a conversation. The fiancé of one of the men has been raped. His friend don't know it. The engaged man asks his friend some silly questions about "fate". He thinks that the trauma they are living as a result of that incident was due to fate. His friend misunderstands him. He thinks he was pissed off because he was not granted tenure on a faculty. So he says:
- You know, tenure's just not all it's cracked up to be. I mean, it's not the answer to life's woes, if that's what you're hoping.
The engaged man, realizing that the conversation is going off point, wants to straighten it up and tries to exemplify himself referring to the book "Appointment in Samarra" in which a guy who sees Death and thinks it's come for him, flees to another city, only to discover that Death is waiting for him there. He says, "fate will find you, no matter where you run."
In response, his friend says:
- Remember your father's saying, "We're all dancing on our graves."