How do i create a trail/trace effect for the moving line in the paint event ?

In the top of the form i have:

PointF _pt = new PointF(0F, 0F);
PointF _pt2 = new PointF(1F, 1F);
PointF _pt3 = new PointF(2F, 2F);
Color _lineColor = Color.FromArgb(0, 255, 0);

Then in the paint event:

private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            anglecounter += 1;
            double x = pictureBox1.Size.Width / 2 + 256 *Math.Cos(anglecounter * Math.PI / 180);
            double y = pictureBox1.Size.Height / 2 +256 * Math.Sin(anglecounter * Math.PI / 180);
            CloudEnteringAlert.Paint(e.Graphics, factor, distance);
            e.Graphics.DrawLine(
            new Pen(Color.Red, 2f),
            new Point(pictureBox1.Size.Width / 2, pictureBox1.Size.Height/2),
            new Point((int)x, (int)y));

            e.Graphics.DrawEllipse(
                new Pen(Color.Red, 2f),
                0, 0, pictureBox1.Size.Width, pictureBox1.Size.Height);

            // create the fade path and gradient
            GraphicsPath gp = new GraphicsPath(FillMode.Winding);
            gp.AddLine(new PointF((float)(pictureBox1.Size.Width / 2), (float)(pictureBox1.Size.Height / 2)),new PointF( (float)x,(float)y));
            gp.AddCurve(new PointF[] { _pt2, _pt3, _pt });
            gp.AddLine(new PointF((float)x, (float)y), new PointF((float)(pictureBox1.Size.Width / 2), (float)(pictureBox1.Size.Height / 2)));
            PathGradientBrush pgb = new PathGradientBrush(gp);
            pgb.CenterPoint = new PointF((float)x, (float)y);
            pgb.CenterColor = Color.FromArgb(128, _lineColor);
            pgb.SurroundColors = new Color[] { Color.Empty };
            // draw the fade path
            e.Graphics.FillPath(pgb, gp);
        }

The first part in the paint event is working.

I draw a circle in the pictureBox1. Then draw a line from the center of the circle to the edge of the circle and move the angle by 1. Since the paint event is called by a timer i had already then i see the line is moving like a radar for example.

Now i want to add to the line the effect of a radar when the line leave behind effect of trail or trace.

So i added the part of the GraphicsPath but i dont see anything of it its not drawing anything this part. I just keep see the curcle and the line moving.

Whats wrong ?