Hi Jacky,
You have to change the content alignment of the label - in this case you need top left alignment. The following code shows a label positioned at the bottom right side of the control, 5 points from the chart right and bottom edges.
NLabel label = new NLabel();
label.Text = "Some Label Text";
label.ContentAlignment = ContentAlignment.TopLeft;
label.Location = new NPointL(new NLength(100, NRelativeUnit.ParentPercentage), new NLength(100, NRelativeUnit.ParentPercentage));
label.UseAutomaticSize = true;
label.Margins = new NMarginsL(5, 5, 5, 5);
nChartControl1.Document.RootPanel.ChildPanels.Add(label);
Hope this helps - let us know if you have any questions or comments.