Почему у меня не строится нормальный график?

Bitmap graphic = new Bitmap(graph.Width, graph.Height);
Graphics graphics = Graphics.FromImage(graphic);
Pen myPen = new Pen(Color.Blue);
float x_min = 1.2F;
float x_max = 6.3F;
float y_min = (float)(5 * Math.Tan(x_min + 7) / Math.Pow(x_min + 3, 2));
float y_max = (float)(5 * Math.Tan(x_max + 7) / Math.Pow(x_max + 3, 2));
float kx = graph.Width / x_max;
float ky = graph.Height / y_max;
float x1 = x_min;
float y1 = y_min;
float x2, y2;
float x_step = 0.2F;
for (float x = x_min; x < x_max; x += x_step)
{
    x2 = x1 + x_step;
    y2 = (float)(5 * Math.Tan(x2 + 7) / Math.Pow(x2 + 3, 2));
    graphics.DrawLine(myPen, x1 * kx, graph.Width - y1 * ky, x2 * kx, graph.Height - y2 * ky);
    x1 = x2;
    y1 = y2;
}
graph.Image = graphic;][1]][1]