If you are using C# 4.0 the Chart object is included in the .NET library, meaning it's free for any use. You can also import the object if you are using .NET 3.5.
I use it and it is pretty decent. As with other charting libraries, I have found it helpful to create a custom control or wrapper that makes it even easier to create commonly used charts.
Here is a link to the sample environment:
http://code.msdn.microsoft.com/mschart[
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