Connection string has not been initialized

<pre>using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Drawing;
using System.Drawing.Drawing2D;
using dotnetCHARTING;
using System.Configuration;



namespace dotnetdrilldown03
{
    public partial class Drilldown4 : System.Web.UI.Page
    {
        
        protected void Page_Load(object sender, EventArgs e)
        {
            Chart.DefaultSeries.ConnectionString = ConfigurationManager.AppSettings["sqlconnection"];
            Chart.Title = "Item sales";
            Chart.XAxis.Label.Text = "Years";
           // Chart.TempDirectory = "temp";
            Chart.Debug = true;

            Chart.DateGrouping = TimeInterval.Years;
            Chart.DrillDownChain = "Years,Quarters,Months,days=Days,hours,minutes,seconds";

            Chart.DefaultSeries.DefaultElement.ToolTip = "%yvalue";

            
            Chart.SeriesCollection.Add();
            Chart.Series.Name = "Items 1";
            Chart.Series.StartDate = new System.DateTime(2016, 1, 1, 0, 0, 0);
            Chart.Series.EndDate = new System.DateTime(2016, 12, 31, 23, 59, 59);
            Chart.Series.SqlStatement = @"SELECT Duration,Sum(Total) FROM Report  WHERE Duration >= #STARTDATE# AND OrderDate <= #ENDDATE#  GROUP BY Report.Duration";

            Chart.SeriesCollection.Add();
        }
       
    }
}