As you say you have real time values for a thing , so for plotting a piechart , i think you need to capture and store the values of property
on datatable or DB . To do this you may need to have a subcription on that thing on datachange of a property for that specfic thing .
Then after getting all values for thing , you may need service which will bind ("All Data") to piechart , you can refer charts in TW documentation
