This is an often-requested feature. The next version of Measurement Studio will contain Annotations as a feature. These will allow you to do a variety of things including placing text, images, and arrows on a graph. You can expect a Beta of the next version of Measurement Studio soon.
If you can't wait for this feature, you have a couple of ways that you can annotate the graph manually. One option is to put a Static control on top of the graph. You can move it programatically if you are trying to annotate a particular point. If your plot is static, you could create a bitmap that contains your annotation and make it the background image of the graph control.
