This requirement is something which is not possible as of now. Maybe you could create an enhancement request for the same. There seem to be a lot of checks which don't allow you enter a non-numeric value for the threshold. Must have designed this way purposefully!
As a dirty workaround, if we remove those checks private editing the final rules, this is how the PRPC generates java when a property is given as a Threshold value for chart control.(check attachment). I've tested giving a work page property. But anyway during execution of that particular code of this section(with a chart control in it), pyworkpage might not be available. Try referring a section parameter and check if that helps.
Even if it works, this would work only for a single section save! Next time after removing the final rule private edits, you'll not be able to save the section with a "parameter" for threshold value.