Custom control not showing up in iOS

(This is a beginner question on Xamarin.Forms - it´s my first attempt creating a custom control :-)

I am trying to create a Xamarin.Forms control for OxyPlot using the Customizing Controls for Each Platform article.

I did the following steps

• created an OxyPlot.XamarinForms.PlotView class based on Xamarin.Forms.View in a portable project.
• created an OxyPlot.XamarinFormsIOS.PlotViewRenderer and added a ExportRendererAttribute in a Xamarin.iOS project.
• created an App class in a portable project
• created an Xamarin.iOS app where the view controller is created from the App and referenced all the other projects

But nothing shows up. What part is missing? Debugging shows that the PlotViewRenderer is never called. If I replace the PlotView by a Button in the App, the button shows up fine. Is something wrong with the ExportRendererAttribute?

The source code is at https://github.com/objorke/oxyplot/tree/features/XamarinForms, see the Source/OxyPlot.XamarinForms.sln solution.