By a “gentle slope”, I mean a goal way easier than you want to set up.
So create new goal → link with Strava → set up a goal with lots of leeway, like this:
Finish setting that up, and go to graph.beeminder.com, and open that goal in the interface. Then I set things up like this, so the end-date is tomorrow and the goal total is 500 calories:
I can submit that graph without an error, then, because it’s making the goal harder, not easier.
You can see the result in this screenshot:
Does that help?
Like I mentioned, Beeminder really isn’t intended to do this (so much so that I’m trying to help you out here as a fellow user, not as Beeminder’s support czar, off the clock – this is not something support can spend time helping you out with). It’s useful to know that this is what you really really want to be able to do, so thanks for sharing the feedback… but it’s not something I anticipate changing anytime soon!