Forgive me for being a little naive perhaps, but it seems that System.Windows.Controls.DataVisualization.Charting has vanished from VS2010, and blend 4. I'm trying to make a bar graph with a line overlayed, but can't even get started because I can't find the appropriate controls.
I know I could use an external graphing package, but I'd like to try the inbuilt controls first.
The original MS Chart controls were never part of the framework. They are an separate project.
You can download them here.
There is also more info on their project page.
There is an addon to the IDE, (here), but it's for VS2008, I don't know if it will work with 2010. I would assume they are going to update it at some point, but there doesn't seem to be any information on when that will happen.
I stand corrected, .Net 4.0 apparently now has built in chart controls. You shouldn't need to install anything. You'll need to make sure you are targeting framework 4.0.
You mention Blend so I assume that you are refering to the WPF/Silverlight chart controls.
The System.Windows.Controls.DataVisualization.Charting namespace is part of the WPF Toolkit. You will have to download this toolkit and reference it from your project.