OxyPlot is an open source plot generation library that is licensed under the MIT license. The MIT license is very permissive and permits use in proprietary software.
The library is based on .NET and targets multiple platforms. The core library is a portable library, which makes it easy to re-use plotting code on different platforms.
The goal has been to provide a plotting model that is simple to use and open for extension. It should be possible to change the appearance of your plots by changing simple properties. More customization can be achieved by deriving new subclasses that have full control of both behaviour and rendering.
The features of the library can be explored in the example applications included in the source code.