Calculating a ratio between fields from separate nodes

I’d like to divide node A’s field foo by node B’s field bar. To be able to eval() them, I should perform a join(), right?

But each node has multiple series distinct by a single tag. And join() appears to remove the tag. How do I join() in a way that preserves the tag so that I can groupby() it and eval() related series?

The answer is to make sure to GROUP BY your desired tag prior to join().