两个Picturebox 对应两个副线程，只运行一个picturebox ，或者两个一起用的话，偶尔会出现Graphics.FromImage，正在使用的错误，System.InvalidOperationException: 对象当前正在其他地方使用。
            在 System.Drawing.Graphics.FromImage(Image image)

    Private Sub BgDrawPicturebox2_DoWork(ByVal sender As System.Object, ByVal e As               System.ComponentModel.DoWorkEventArgs) Handles BgDrawPicturebox2.DoWork

             PictureBox2.Refresh()

            Dim g3 As Graphics
            Dim bmp3 As Bitmap
            bmp3 = New Bitmap(470, 240)
            PictureBox2.Image = bmp3
            g3 = Graphics.FromImage(PictureBox2.Image)

            DrawOrdinate2(g3, 1, 5, 35, 220, 435, 20)
            DrawaxisValue2(g3)
            Drawtarget2(1, g3)

            g3.Dispose()
            System.Threading.Thread.Sleep(300)

End Sub