Chart Doesn't Reflect Data

All the sample data is correctly formatted for the stack chart when it is set as the items source, why is the chart display all wrong, what did I do wrong?

C#:

public partial class MainPage : UserControl
    {
        public MainPage()
        {
            var costs = new List<CostInfoCustom>();

            costs.Add(new CostInfoCustom { Qty = 6, CostDate = new DateTime(2012, 02, 01), TaskInfoKey = new TaskInfo { TaskNo = 3 } });
            costs.Add(new CostInfoCustom { Qty = 2, CostDate = new DateTime(2012, 03, 04), TaskInfoKey = new TaskInfo { TaskNo = 1 } });
            costs.Add(new CostInfoCustom { Qty = 2, CostDate = new DateTime(2012, 02, 01), TaskInfoKey = new TaskInfo { TaskNo = 2 } });
            costs.Add(new CostInfoCustom { Qty = 0, CostDate = new DateTime(2012, 02, 01), TaskInfoKey = new TaskInfo { TaskNo = 2 } });
            costs.Add(new CostInfoCustom { Qty = 0, CostDate = new DateTime(2012, 05, 03), TaskInfoKey = new TaskInfo { TaskNo = 3 } });
            costs.Add(new CostInfoCustom { Qty = 0, CostDate = new DateTime(2012, 02, 01), TaskInfoKey = new TaskInfo { TaskNo = 1 } });
            costs.Add(new CostInfoCustom { Qty = 6, CostDate = new DateTime(2012, 02, 01), TaskInfoKey = new TaskInfo { TaskNo = 1 } });
            costs.Add(new CostInfoCustom { Qty = 5, CostDate = new DateTime(2012, 06, 02), TaskInfoKey = new TaskInfo { TaskNo = 1 } });
            costs.Add(new CostInfoCustom { Qty = 5, CostDate = new DateTime(2012, 02, 03), TaskInfoKey = new TaskInfo { TaskNo = 2 } });
            costs.Add(new CostInfoCustom { Qty = 5, CostDate = new DateTime(2012, 02, 04), TaskInfoKey = new TaskInfo { TaskNo = 3 } });
            costs.Add(new CostInfoCustom { Qty = 5, CostDate = new DateTime(2012, 02, 04), TaskInfoKey = new TaskInfo { TaskNo = 2 } });
            costs.Add(new CostInfoCustom { Qty = 5, CostDate = new DateTime(2012, 07, 07), TaskInfoKey = new TaskInfo { TaskNo = 1 } });
            costs.Add(new CostInfoCustom { Qty = 5, CostDate = new DateTime(2012, 02, 02), TaskInfoKey = new TaskInfo { TaskNo = 3 } });
            costs.Add(new CostInfoCustom { Qty = 3, CostDate = new DateTime(2012, 02, 05), TaskInfoKey = new TaskInfo { TaskNo = 1 } });
            costs.Add(new CostInfoCustom { Qty = 1, CostDate = new DateTime(2012, 10, 02), TaskInfoKey = new TaskInfo { TaskNo = 2 } });
            costs.Add(new CostInfoCustom { Qty = 1, CostDate = new DateTime(2012, 02, 01), TaskInfoKey = new TaskInfo { TaskNo = 3 } });

            SetChartData(costs);
        }

        private void SetChartData(List<CostInfoCustom> costs)
        {
            InitializeComponent();

            //Set the X origin to 0
            HourChart.YAxes.Add(new XYDecimalAxis { Minimum = 0 });

            //Group costs by date and get all the task numbers from the data
            var costsByDates = costs.GroupBy(c => c.CostDate);
            var allTaskNumbers = costs.Select(x => x.TaskInfoKey.TaskNo);

            //For each day add qty = 0 records where a task is missing
            foreach (var costsByDay in costsByDates)
            {
                var missingTaskNumbers = allTaskNumbers.Except(costsByDay.Select(x => x.TaskInfoKey.TaskNo)).ToList();
                foreach (int taskNumber in missingTaskNumbers)
                {
                    costs.Add(new CostInfoCustom { Qty = 0, CostDate = costsByDay.Key, TaskInfoKey = new TaskInfo { TaskNo = taskNumber } });
                }
            }

            //group the costs by task
            var costsGroupedByTaskNo = costs.GroupBy(c => c.TaskInfoKey.TaskNo);

            //for each task's costs, create a data series.
            foreach (var taskCosts in costsGroupedByTaskNo)
            {
                Debug.WriteLine("TaskNo: " + taskCosts.Key + " - Total Hours: " + taskCosts.Sum(x => x.Qty) + " On Days: " + string.Join(",", taskCosts.Select(cg => cg.CostDate.Day).Distinct()));
                HourChart.Series.Add(new BarSeries
                {
                    ItemsSource = taskCosts.ToList(),
                    YPath = "Qty",
                    XPath = "CostDate",
                    StackKind = XYSeriesStackKind.Normal
                });
            }
        }
    }

 XAML:

<UserControl
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:controls="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls"
  xmlns:charts="http://schemas.actiprosoftware.com/winfx/xaml/charts" 
  xmlns:controlsToolkit="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Toolkit"
  xmlns:chartingToolkit="clr-namespace:System.Windows.Controls.DataVisualization.Charting;assembly=System.Windows.Controls.DataVisualization.Toolkit"
  x:Class="HoursChart.MainPage">

    <UserControl.Resources>
        <Style TargetType="charts:XYChart">
            <Setter Property="Width" Value="1000" />
            <Setter Property="Height" Value="900" />
        </Style>
    </UserControl.Resources>

    <Grid>
        <charts:XYChart Name="HourChart"/>
    </Grid>

</UserControl>

 Console Output:

TaskNo: 3 - Total Hours: 17 On Days: 1,3,4,2,7,5
TaskNo: 1 - Total Hours: 21 On Days: 4,1,2,7,5,3
TaskNo: 2 - Total Hours: 13 On Days: 1,3,4,2,7,5

Chart Output: