如何重绘图表？

Question

我的程序只打印图表。 如果我在屏幕外移动表单，则不会打印图表。我该如何解决？我是一个初学者。 你能帮忙吗？ 谢谢。

    private void button1_Click(object sender, EventArgs e)
    {
       Graphics gr ;
       int m11, m12, m21, m22, dx, dy, ax, ay;
        gr = pictureBox1.CreateGraphics();
        int xmin = -5;
        int xmax = 10;
        int ymin = 0;
        int ymax = xmax * xmax;

        ax = pictureBox1.Size.Width / (xmax - xmin);
        ay = pictureBox1.Size.Height / (ymax - ymin);
        m11 = ax;
        m12 = 0;
        m21 = 0;
        m22 = -ay;
        dx = -xmin * ax;
        dy = pictureBox1.Size.Height - ay * (-ymin);

        System.Drawing.Drawing2D.Matrix M = new 
        System.Drawing.Drawing2D.Matrix(m11, m12, m21, m22, dx, dy);
        gr.Transform = M;
        int x = 0;
        for (x = xmin; x < xmax; x++)
        {
            System.Drawing.Point p1 = new System.Drawing.Point(x, x * x);
            System.Drawing.Point p2 = new System.Drawing.Point(x + 1, (x + 1) * (x + 1));
            float brushSize = 0.2F;
            System.Drawing.Pen pen = new 
            System.Drawing.Pen(System.Drawing.Brushes.Black, brushSize);
            gr.DrawLine(pen, p1, p2);

        }