You can draw circles and rectangles on the chart, about the same way you draw a line

Either use the Lines menu, or right click on the graph, and select Circle or Rectangle.

A circle/rectangle is defined by its diagonal. Just click to define the two extremes point.

Screen Shot 2013-02-23 at 7.20.31 PM

Once a circle or rectangle is defined, right-clicking on it displays a context menu similar to the ones used for lines.

Screen Shot 2013-02-23 at 7.23.58 PM