Bar charts only support categorical X-Axis, so regardless of your X-Values, the attachment position must be expressed as an index in the X-Axis (0 for the first point, 1 for the second, etc.). In a line chart, the X-Axis is not categorical (if X-Values are passed) and therefore the attachment position is dependent on the X-Value and not on the point index.
ChartReport.Gallery = Gallery .Lines ;
Annotations annots = new Annotations ();
ChartReport.Extensions.Add(annots);
annots.EnableUI = true ;
annots.ToolBar.Visible = true ;
AnnotationText text = new AnnotationText ( "Katrina" );
text.Attach(2, 10);
text.Color = Color .LightBlue;
text.PlotAreaOnly = true ;
text.AllowMove = false ;
annots.List.Add(text);