Annotations allow you add custom text blocks to the plot area of the chart. They can be single or multilane and have different look and fill. The annotation location can be fixed (given as fixed coordinates in pixels) or relative (given as percentage of the plot area size). The relative location allows create some kind of binding of the annotation to the category.
You can use Drag & Drop technique to move annotation to the desired location. You can change annotation properties using context menu that appears when annotation is right clicked, or using API.