There isn't a way to change this at this time.
It mostly is a function of how that graph displays liabilities in there as well. Even if you have $0, it will still graph that.
I don't know future plans, but it was explained that these home screens panels are more 1.0 releases with more features planned. They have already improved upon some of the others' initial releases.
You are not alone in you desire for the ability to edit.