There is OxyPlot which I recommend. It has packages for WPF, Metro, Silverlight, Windows Forms, Avalonia UI, XWT. Besides graphics it can export to SVG, PDF, Open XML, etc. And it even supports Mono and Xamarin for Android and iOS. It is actively developed too.
There is also a new (at least for me) open source .NET plotting library called Live-Charts. The plots are pretty interactive. Library suports WPF, WinForms and UWP. Xamarin is planned. The design is made towards MV* patterns.
But @Paweł Audionysos suggests not such a good performance of Live-Charts WPF.