One thing you could theoretically do is put in an event for no update X minutes, and give that event a handler that sets the point to 0. But, this would mix the zeros in with your data and possibly cause other problems if someone mistakes it for real. In DGLux you can write a script that combs the source value table and transposes it into another table, interpolating however you like. For the current built-in charts, I don't believe there is a solution like the second.
