在面板上画线

我试着做一个图形。当我点击标签时，我想画一条线。它可以工作，它画出我的线，但在最后一点，在左上角有另一条线…我不知道为什么。

(这是无用的，但它是为另一个项目，我试图了解如何工作的图纸)

下面是我的代码:

public partial class Form1 : Form
{
    Pen myPen = new Pen(Color.Blue);
    Graphics g = null;
    int start_x = 0, start_y;
    Point[] points = new Point[1000];
    int test = 0;
    public Form1()
    {
        InitializeComponent();
        start_y = canvas.Height / 2;
        points[0] = new Point (start_x,start_y);
        myPen.Width = 1;
    }
    private void drawLine()
    {
        g.DrawLines(myPen, points);
    }
    private void incrementation_Click(object sender, EventArgs e)
    {
        test = test + 1;
        incrementation.Text = test.ToString();
        if(test == 1)
        {
            points[1] = new Point(100, start_y);
        }
        if (test == 2)
        {
            points[test] = new Point(200, 90),new Point(220, 10);
        }
        if (test == 3)
        {
            points[test] = new Point(220, 10);
            drawLine();
        }
    }
    private void canvas_Paint(object sender, PaintEventArgs e)
    {
        g = canvas.CreateGraphics();
    }
}