Much sadness.
Any solution that involves fake data is wrong, and I say that without being QS-obsessive. It’s like getting the affordances wrong in physical design; e.g. doors with pull-handles shouldn’t be push-to-open.
This example is tied to the whole world of how Beeminder (currently) fails to celebrate success. Seems closely related to the uncle button, in functionality if not intent.
Happily, @rebeccayoung wasn’t shy about emailing support. 
Also, sometimes it’s nice to leave a completed goal in your trophy case for a while. Now, there’s a #UVI idea to replace back-burnering! Like feature-my-goal, but personal.