Thanks for the bug report! I see this is happening also in the examples in the Windows Forms Example Browser (it is working in WPF). I have added issue
https://oxyplot.codeplex.com/workitem/10120
(I don't have time to solve this myself at the moment)
https://oxyplot.codeplex.com/workitem/10120
(I don't have time to solve this myself at the moment)