Seems like a bit of a bug to me that you can't reference an existing label in a series! I tried the approach you suggested and I got it to work.
But I've now got a serious issue as a result. The problem is that if I actually add a label, with the last line of my code: oChartSeries.Labels.Add(label)
The resulting pptx file crashes when you open it and try and save it! If I comment out that last line (i.e. it doesn't do anything to that chart) it's fine.
Here's my code and I have added my generated file:
Dim oChartCategoryCollection As ChartCategoryExCollection = oGraph.ChartData.Categories
Dim iPos As Integer
iPos = 4
Dim CatValue As ChartCategoryEx
CatValue = oChartCategoryCollection(iPos)
Dim Val As Object
Val = CatValue.ChartDataCell.Value
Dim oChartSeries As ChartSeriesEx
oChartSeries = oGraph.ChartData.Series(1)
Dim label As New DataLabelEx(oChartSeries)
label.TextFrame.Text = Val.ToString
label.Id = iPos