easiest way:

            BoxAnnotation anno = new BoxAnnotation();
            anno.TextStyle.FontColor = Color.Black;
            anno.Text = "SEAL OF APPROVAL";
            anno.TextStyle.Font = new Font("Arial", 30f, FontStyle.Bold);
            anno.TextStyle.HorizontalAlign = StringAlignment.Near;
            anno.TextStyle.VerticalAlign = StringAlignment.Near;
            anno.Location.Type = LocationType.Percentage;
            anno.Location.LocationX = anno.Location.LocationY = 50;
            anno.PE.StrokeOpacity = 0;
         
            ch.Annotations.Add(anno);

best way:

void ch_FillSceneGraph(object sender, FillSceneGraphEventArgs e)
        {
            Rectangle bounds = e.ChartCore.GetBorderLayer().OuterBound;
            Path pathPrimitive = new Path();
            pathPrimitive.GraphicsPath = new GraphicsPath();
            string textString = "SEAL OF APPROVAL";
            pathPrimitive.GraphicsPath.AddString(textString, new FontFamily("Arial"), (int)FontStyle.Bold, 30f, new Point(0, 0), StringFormat.GenericDefault);
            pathPrimitive.PE.ElementType = PaintElementType.Hatch;
            pathPrimitive.PE.Hatch = FillHatchStyle.Sphere;
            pathPrimitive.PE.Fill = Color.Red;
            pathPrimitive.PE.FillStopOpacity = 0;

            SizeF pathSize = pathPrimitive.GraphicsPath.GetBounds().Size;
            GraphicsContext translate = new GraphicsContext();
            translate.Transform = new Matrix();
            translate.Transform.Translate(bounds.Left + bounds.Width / 2f - pathSize.Width / 2f, bounds.Top + bounds.Height / 2f - pathSize.Height / 2f);
            e.SceneGraph.Add(translate);
            e.SceneGraph.Add(pathPrimitive);
            translate = new GraphicsContext();
            translate.ResetTransform = true;
            e.SceneGraph.Add(translate);
        }