График при помощи Graphics C#

private double F(double x)
        {
            return 1 / 2 * Math.Sin(x) + 4;
        }
 private void button1_Click(object sender, EventArgs e)
        {
Bitmap image = new Bitmap(100, 100, System.Drawing.Imaging.PixelFormat.Format32bppRgb);
            System.Drawing.Graphics.FromImage(image).Clear(Color.White);
            pictureBox1.Image = image;
            Pen pen = new Pen(Color.Red, 1);
            for (int i = -40; i < 40; i++)
            {
                System.Drawing.Graphics.FromImage(image).DrawLine(pen, 50 + i, Convert.ToInt32(50 - F(i)), 51 + i, Convert.ToInt32(50 - F(i + 1)));
            }
            pen.Dispose();
            pictureBox1.Invalidate();}