expression.ShowPercentage
expression Required. An expression that returns one of the objects in the Applies To list.
Remarks
The chart must first be active before you can access the data labels programmatically.
Example
This example enables the percentage value to be shown for the data labels of the first series on the first chart.
Sub UsePercentage()
ActiveSheet.ChartObjects(1).Activate
ActiveChart.SeriesCollection(1) _
.DataLabels.ShowPercentage = True
End Sub