await graph.UpdateDataSources ();
((PlotView)View).Model.Update ();
View.SetNeedsDisplay ();

((PlotView)View).Model.InvalidatePlot (true);
View.SetNeedsDisplay ();

        // Function to plot data
        private void plotData(double numWeeks, double startingSS)
        {

            // Initialize new Salt Split class for acess to data variables
            Salt_Split_Builder calcSS = new Salt_Split_Builder();
            calcSS.compute(numWeeks, startingSS, maxDegSS);

            // Create the OxyPlot graph for Salt Split
            OxyPlot.Wpf.PlotView plot = new OxyPlot.Wpf.PlotView();
            
            var model = new PlotModel();

            // Add Chart Title
            model.Title = "Salt Split Degradation";

            // Create new Line Series
            LineSeries linePoints = new LineSeries() { StrokeThickness = 1, MarkerSize = 1, Title = numWeeks.ToString() + " weeks" };


            // Add each point to the new series
            foreach (var point in calcSS.saltSplitCurve)
            {
                DataPoint XYpoint = new DataPoint();
                XYpoint = new DataPoint(point.Key, point.Value * 100);
                linePoints.Format("%", XYpoint.Y);
                linePoints.Points.Add(XYpoint);
            }

            listPointAray.Add(linePoints);


            // Define X-Axis
            var Xaxis = new OxyPlot.Axes.LinearAxis();
            Xaxis.Maximum = numWeeks;
            Xaxis.Minimum = 0;
            Xaxis.Position = OxyPlot.Axes.AxisPosition.Bottom;
            Xaxis.Title = "Number of Weeks";
            model.Axes.Add(Xaxis);

            //Define Y-Axis
            var Yaxis = new OxyPlot.Axes.LinearAxis();
            Yaxis.MajorStep = 15;
            Yaxis.Maximum = calcSS.saltSplitCurve.Last().Value * 100;
            Yaxis.MaximumPadding = 0;
            Yaxis.Minimum = 0;
            Yaxis.MinimumPadding = 0;
            Yaxis.MinorStep = 5;
            Yaxis.Title = "Percent Degradation";
            model.Axes.Add(Yaxis);

            // Add Each series to the
            foreach (var series in listPointAray)
            {
                LineSeries newpoints = new LineSeries();
                newpoints = linePoints;
                model.Series.Add(newpoints);
            }


            // Add the plot to the window
            plot.Model = model;
            SaltSplitChartGrid.Children.Add(plot);
            
        }

The element cannot be added, it already belongs to a Plot Model

"http://stackoverflow.com/questions/24938689/how-to-plot-multiple-lineseries-on-an-oxyplot-chart/24942582#24942582"

        PlotModel model = new PlotModel("HighLowSeries") { LegendSymbolLength = 24 };

        DateTimeAxis dateAxis = new DateTimeAxis(AxisPosition.Bottom, "Date", "MM/dd") { MajorGridlineStyle = LineStyle.Solid, MinorGridlineStyle = LineStyle.Dot };
        model.Axes.Add(dateAxis);
        LinearAxis valueAxis = new LinearAxis(AxisPosition.Left, 0) { MajorGridlineStyle = LineStyle.Solid, MinorGridlineStyle = LineStyle.Dot, Title = "Price" };
        model.Axes.Add(valueAxis);

        DateTime dt = DateTime.Now;

        var s1 = new HighLowSeries("random values")
        {
            Color = OxyColors.Black,
        };
        var r = new Random();
        var price = 100.0;
        for (int x = 0; x < 24; x++)
        {
            price = price + r.NextDouble() + 0.1;
            var high = price + 10 + r.NextDouble() * 10;
            var low = price - (10 + r.NextDouble() * 10);
            var open = low + r.NextDouble() * (high - low);
            var close = low + r.NextDouble() * (high - low);
            s1.Items.Add(new HighLowItem(DateTimeAxis.ToDouble(dt.AddDays(x)), high, low, open, close));
        }
        model.Series.Add(s1);

<oxy:Plot Grid.Row="1" Model="{Binding ModelPower}" >
            <oxy:Plot.Axes>
                <oxy:LinearAxis Position="Left" Title="Power" StartPosition="0.5" EndPosition="1" Key="Power"/>
                <oxy:LinearAxis Position="Left" Title="Price" StartPosition="0" EndPosition="0.5" Key="Price"/>
                <oxy:DateTimeAxis Position="Bottom" EndPosition="1" StartPosition="0" StringFormat="hh:mm:ss.fff" IntervalType="Seconds"/>
            </oxy:Plot.Axes>
            <oxy:Plot.Series>
                <oxy:LineSeries ItemsSource="{Binding DataCollection}" MarkerType="Circle" Color="Blue" YAxisKey="Power" DataFieldX="DateTime" DataFieldY="APower"/>
                <oxy:LineSeries ItemsSource="{Binding DataCollection}" MarkerType="Circle" Color="Green" YAxisKey="Power" DataFieldX="DateTime" DataFieldY="BPower"/>

                <oxy:LineSeries ItemsSource="{Binding DataCollection}" MarkerType="Circle" Color="Blue" YAxisKey="Price" DataFieldX="DateTime" DataFieldY="BestA"/>
                <oxy:LineSeries ItemsSource="{Binding DataCollection}" MarkerType="Circle" Color="Green" YAxisKey="Price" DataFieldX="DateTime" DataFieldY="BestB"/>
            </oxy:Plot.Series>
        </oxy:Plot>