Recently I finish a .NET 3.5 based desktop application, where I need to make graphs. For this I search few open source, ready to use library and with help of SourceForge.net, I am able to find one such good library called ZEDGraph. However, it seems this project is no longer active, but it has a good reason that they might not have any new thing to add as it seems already full of features.
You can go to ZedGraph Wiki, or can ask question on its sourceforge forum which still is somewhat active and people are posting there.
This is really a good project and I will stick with it as I modify its source as well for one of very own custom requirement and hence I know its source architecture to us it in future as well.