Simply use DataPoint's LegendText
property when populating chart's data:
PieChart.Series[0].Points.Add(new DataPoint(1, 1) { LegendText = "aaa" });
PieChart.Series[0].Points.Add(new DataPoint(1, 2) { LegendText = "bbb" });
PieChart.Series[0].Points.Add(new DataPoint(1, 3) { LegendText = "ccc" });
Alternative way is to set LegendText
each time you set Label
:
point.Label = "Value: #VAL";
point.LegendText = "Series#INDEX"; // Or whatever you want