在datagridview中画一根线，线的终点超出datagridview显示范围，点击滚动条，线条无法保持原样（附代码）

在datagridview中添加一根线，线的终点超出datagridview显示范围，点击滚动条，线条无法保持原样（附代码）
如何使得线条保持原样？
还请各位帮忙看看，谢谢了。


using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

private void Form1_Load(object sender, EventArgs e)
{

for (int i = 0; i < 20; i++)
{
dataGridView1.Columns.Add(i.ToString(),"clo"+i.ToString());
}
dataGridView1.Rows.Add(50);
for (int i = 0; i < dataGridView1.Rows.Count; i++)
{
dataGridView1.Rows[i].HeaderCell.Value = "PD" + i.ToString();
}
}

private void dataGridView1_Paint(object sender, PaintEventArgs e)
{
Pen p = new Pen(Color.Black, 2);

int startX;
int startY;
int endX;
int endY;

startX = dataGridView1.Rows[1].HeaderCell.Size.Width;
startY = dataGridView1.Columns[1].HeaderCell.Size.Height;

endX = dataGridView1.Size.Width;
endY = dataGridView1.Size.Height;

Point start = new Point(startX, startY);
Point end = new Point(endX, endY);
e.Graphics.DrawLine(p, start, end);
}
}
}