Web (ASP.NET C#) เลือกเปลี่ยนปีใน DropDownList แล้วข้อมูลไม่ยอมเปลี่ยนตามคับ

<asp:Content ID="Content1" ContentPlaceHolderID="MainContent" runat="server">
    <br />
    <div>


        <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True"></asp:DropDownList>


        <script type="text/javascript" src="https://www.google.com/jsapi"></script>  
        <asp:GridView ID="gvData" runat="server">  
        </asp:GridView>  
        <br />  
        <br />  
        <asp:Literal ID="ltScripts" runat="server"></asp:Literal>  
        <div id="chart_div" style="width: 660px; height: 400px;">  


        </div>  
    </div>  
</asp:Content>

#region " [ Using ] "  
using System;
using System.Web.UI;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;
using System.Text;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI.WebControls;
using System.Text.RegularExpressions;
#endregion

namespace WebApplication4
{
    public partial class WebForm2 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {

                DropDownList1.Items.Add(new ListItem("2016", "2016"));
                DropDownList1.Items.Add(new ListItem("2015", "2015"));
                DropDownList1.Items.Add(new ListItem("2014", "2014"));
                // Bind Gridview  
                BindGvData();

                // Bind Charts  
                BindChart();

                

               
            }
        }

        private void BindGvData()
        {
            gvData.DataSource = GetChartData();
            gvData.DataBind();
        }

        private void BindChart()
        {
            DataTable dsChartData = new DataTable();
            StringBuilder strScript = new StringBuilder();

            try
            {
                dsChartData = GetChartData();

                strScript.Append(@"<script type='text/javascript'>  
                    google.load('visualization', '1', {packages: ['corechart']});</script>  
  
                    <script type='text/javascript'>  
                    function drawVisualization() {         
                    var data = google.visualization.arrayToDataTable([  
                    ['week', 'OpenQty', 'ConfirmQty'],");

                foreach (DataRow row in dsChartData.Rows)
                {
                    strScript.Append("['" + row["week"] + "'," + row["OpenQty"] + "," +
                        row["ConfirmQty"] + "],");
                }
                strScript.Remove(strScript.Length - 1, 1);
                strScript.Append("]);");

                strScript.Append("var options = { title : 'PO Open And Confrim ', vAxis: {title: 'Qty'},  hAxis: {title: 'week'}, seriesType: 'bars' };");//กำหนดชื่อต่างๆ
                strScript.Append(" var chart = new google.visualization.ComboChart(document.getElementById('chart_div'));  chart.draw(data, options); } google.setOnLoadCallback(drawVisualization);");
                strScript.Append(" </script>");

                ltScripts.Text = strScript.ToString();
            }
            catch
            {
            }
            finally
            {
                dsChartData.Dispose();
                strScript.Clear();
            }
        }

        /// <summary>  
        /// fetch data from mdf file saved in app_data  
        /// </summary>  
        /// <returns>DataTable</returns>  
        public  DataTable GetChartData()
        {
            DataSet dsData = new DataSet();
            try
            {
                SqlConnection sqlCon = new SqlConnection(ConfigurationManager.ConnectionStrings["SVI_MMmoduleEntities"].ConnectionString);
                SqlCommand cmd = new SqlCommand("data_week", sqlCon);
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@Year", Int32.Parse(DropDownList1.SelectedValue));


                SqlDataAdapter sqlCmd = new SqlDataAdapter(cmd);

                sqlCon.Open();

                sqlCmd.Fill(dsData);

                sqlCon.Close();

            }
            catch
            {
                throw;
            }
            return dsData.Tables[0];
        }
    }
}