No worries! Our experience is that the hardest part (and it's not trivial) is associating the "anonymous user ID" to the newly logged-in user. Segment has an identify() function (I think Mixpanel has something similar), where you link the aliases, which then makes the reporting work.

I've not tried June, so I can't say for sure, but it's a pretty common feature for product analytics. I'll be surprised if it's not possible.