OxyPlot is an open source plotting library for .NET.
The code is licensed under the very liberal MIT license. This means that the library can be used in commercial applications, and modified as much as you want. But remember there is no warranty or support included...
The core library is a Portable Class Library which is shared between different user interface platforms. Custom controls are implemented for WPF, Windows 8, Windows Forms, Silverlight, GTK#, Xamarin.iOS and Xamarin.Android.
OxyPlot contains many different axis and series types. If you need some functionality that is not included in the library, you can create a derived class and override the rendering implementation.
It should not take much time to get a plot into your application. Use the NuGet package manager to add the OxyPlot references. Add a "Plot" control to your user interface. Then create a "PlotModel" and bind or assign this to the control.
This project is continuously evolving and any kind of help is greatly appreciated. It could be writing documentation, testing, coding or helping out on the discussion forum!