Обычный конус

            Bitmap bmp = new Bitmap(pictureBox1.Width, pictureBox1.Height);
            Graphics grpah = Graphics.FromImage(bmp);
            Pen pen = new Pen(Color.Blue);
            grpah.DrawLine(pen, 20, 200, 100, 20);
            grpah.DrawLine(pen, 100, 20, 200, 200);
            grpah.DrawCurve(pen, new Point[] {new Point(20, 200),
                new Point(110, 230), new Point(200, 200)}, 0.6f);
            Brush dotted = new SolidBrush(Color.Gray);
            grpah.DrawCurve(new Pen(dotted, 1.0f), new Point[] {new Point(20, 200),
                new Point(110, 170), new Point(200, 200)}, 0.6f);
            pictureBox1.Image = bmp;