||I have the same issue. I suspect it's because I used to have some problems enabled for some "All about the Star Point" lectures, but as far as I can tell those don't exist at all any more, so I'm permanently stuck with my stats showing when those problems are due for review (which is way in the past) without a way to review or delete them. A bug fix would be nice of course but it's definitely non-urgent since it affects nothing else except that graph.
I haven't tested this, but maybe you might get the same result if you learn some of a problem set and then disable it, and those disabled problems come up for review. Do you think that might be the reason in your case, or do you think it's something more like what happened for me?