I have attached a simple project that demonstrates how to get this working. A label template is used (similar to your label template). You can see that as long as the underlying business item has a Percent property - the Percent is displayed in the label. Do run the project and see if you get the same behavior on your side. Then you can see what are the differences between the attached project and your actual application.
I see that you are using a relative source binding. Due to the layout logic of the chart, such a binding is not expected to work and will result in an incorrectly positioned label. I suggest you remove this and use another approach.
Let us know how it goes and if you need more information. If you need further assistance, please modify the attachment and send it back to us so that we can start looking for solutions.
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