Current OxyPlot Version not working with current Xamarin.Forms Version

Hallo,
the current OxyPlot-Version is not working on all Plattforms.
On Android everything works well, but on WindowsPhone and iOS the PlotModel does not appear.
The PlotView appears and displays the Backgroundcolor.
OxyPlot-Version: 2015.1.889-alpha (from nuget)
Xamarin.Forms Version: 1.4.3.6374 (from nuget)

Code for creating the page with graph : (From the Demoproject)

var plotModel = new PlotModel
            {
                Title = "OxyPlot in Xamarin.Forms",
                Subtitle = string.Format("OS: {0}, Idiom: {1}", Device.OS, Device.Idiom),
                Background = OxyColors.LightYellow,
                PlotAreaBackground = OxyColors.LightGray
            };
            var categoryAxis = new CategoryAxis { Position = AxisPosition.Bottom };
            var valueAxis = new LinearAxis { Position = AxisPosition.Left, MinimumPadding = 0 };
            plotModel.Axes.Add(categoryAxis);
            plotModel.Axes.Add(valueAxis);
            var series = new ColumnSeries();
            series.Items.Add(new ColumnItem { Value = 3 });
            series.Items.Add(new ColumnItem { Value = 14 });
            series.Items.Add(new ColumnItem { Value = 11 });
            series.Items.Add(new ColumnItem { Value = 12 });
            series.Items.Add(new ColumnItem { Value = 7 });
            plotModel.Series.Add(series);
            return new ContentPage
            {
                Padding = new Thickness(0, 20, 0, 0),
                Content = new PlotView
                {
                    Model = plotModel,
                    VerticalOptions = LayoutOptions.Fill,
                    HorizontalOptions = LayoutOptions.Fill,
                },
            };

Any idea?