Edit Pie Chart Label

I've created a pie chart to show percentage of attendance for Members v. Non-Members, based off Productions in Tessitura. Non-Members are showing as [none]. Is there a way to change the label for data within a (I don't know the correct term here) category...item? ([none] below should be Non-Members in the example below.) I know how to change the name of the category, but not the data within the category.

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