Areaseries: Fill area below baseline (realtime)

/// <summary>
/// Setup Plot
/// </summary>
private void SetupPlot()
{
    this.time = DateTime.Now;

    this.Plot = new PlotModel();

    this.Plot.Axes.Add(new LinearAxis() 
                                    { 
                                      IsZoomEnabled = false, 
                                      Maximum = 102, 
                                      Minimum=0, 
                                      MajorGridlineStyle = LineStyle.Solid, 
                                      MinorGridlineStyle = LineStyle.Dot, 
                                      Position = AxisPosition.Left 
                                    });

    this.Plot.Axes.Add(new DateTimeAxis()
                                    {
                                        IsZoomEnabled = false,
                                        Position = AxisPosition.Bottom,
                                    });

    var areaSeries = new AreaSeries()
    {
        StrokeThickness = 2,
        LineStyle = OxyPlot.LineStyle.Solid,
        Color = OxyColors.Blue,
        Color2 = OxyColors.Transparent,
        Fill = OxyColor.FromRgb(214, 231, 242),
        DataFieldX2 = "X",
        ConstantY2 = 0
    };

    // Fill series with initial values
    for (int i = 0; i < 60; i++)
    {
        areaSeries.Points.Add(new DataPoint(DateTimeAxis.ToDouble(time.Subtract(new TimeSpan(0, 0, 60 - i))), 0));
    }

    this.Plot.Series.Add(areaSeries);
}

void timer_Tick(object sender, EventArgs e)
{
    var areaSeries = (AreaSeries)this.Plot.Series[0];

    if (areaSeries.Points.Count > 60)
    {
        areaSeries.Points.RemoveAt(0);
    }
    
    // Update-Plot
    double percentage = GetPercentage();

    areaSeries.Points.Add(new DataPoint(DateTimeAxis.ToDouble(this.time), percentage));
    time = time.AddSeconds(1);
    
    this.Plot.InvalidatePlot(true);
}