* if you see a zero, it means there was some activity from some agent type on that day, just not the particular agent type you're looking at

* if you see no data, or that error, then there was no activity at all on that day

To create more consistent behavior here, we would have to run additional queries to check whether there's a real problem or just this scenario.  And that's inefficient (we don't have tons of resources to work with).  Hope this helps :)