VB.NET高手请指教呀！

源码如下：
Imports System.Drawing
Public Class Form1
    Dim g As Graphics = Me.CreateGraphics
    Private Sub Form1_Shown(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Shown
        Dim mypen As Pen = New Pen(Color.Black, 2)
        g.DrawEllipse(mypen, 50, 50, 300, 200)

    End Sub
End Class
在我的电脑上只显示出大半个椭圆，只要是X大于300就显示不出完整的图形，如下面的语句
        g.DrawLine(mypen, 50, 50, 400, 50)
        g.DrawLine(mypen, 50, 250, 300, 250)
绘出的直线一样长，为什么？我试了多次，只要X大于300就显示不出。