OxyPlot disappear after RefreshPlot(true)

Hi ALL, I have a program which takes OxyDataItem and plot it using LineSeries (DateTime Axis vs LinearAxis). However once I have used the function on RefreshPlot(true) the plot seems to disappear. Any idea why would this happen?


public void MyOxyPlot(string GraphName, string GraphType, DateTime StartDate, DateTime EndDate)

    {

        var tmp = new PlotModel(GraphName + " " + GraphType);
        tmp.Axes.Add(new OxyPlot.Axes.LinearAxis(OxyPlot.Axes.AxisPosition.Left) { MajorGridlineStyle = LineStyle.Solid, MinorGridlineStyle = LineStyle.Dot });
        tmp.Axes.Add(new OxyPlot.Axes.DateTimeAxis(OxyPlot.Axes.AxisPosition.Bottom, StartDate, EndDate, null, null, OxyPlot.Axes.DateTimeIntervalType.Hours)
        {
            MajorGridlineStyle = LineStyle.Solid,
            Angle = 90,
            StringFormat = "yyyy",
            MajorStep = 365.0,
            //MajorStep = 1.0 / 24 / 2, // 1/24 = 1 hour, 1/24/2 = 30 minutes
            IsZoomEnabled = true,
            MaximumPadding = 0,
            MinimumPadding = 0,
            //TickStyle = TickStyle.None
        });
        var ls = new OxyPlot.Series.LineSeries("Line1") { DataFieldX = "X", DataFieldY = "Y" };
        List<OxyDataItem> ii = new List<OxyDataItem>();
        foreach (DataRow dtr in TSDataTable.Rows)
        {
            if ((DateTime)dtr["Time Stamp"] >= StartDate && (DateTime)dtr["Time Stamp"] <= EndDate)
                ii.Add(new OxyDataItem { X = (DateTime)dtr["Time Stamp"], Y = (double)dtr[GraphType] }); 
        ls.ItemsSource = ii;
        tmp.Series.Add(ls);
        if (MyMDDPlotModel == null)
        {   
            MyMDDPlotModel = tmp;
            MyMDDPlotModel.InvalidatePlot(false);
            MyMDDPlotModel.RefreshPlot(true);
        }
    }