I\'m creating graphs using JFreeChart:
The problem should be fairly clear.
I suggest using a second series with only one single value in it. For this second series you need to enable the drawing of shapes using the setSeriesShapesVisible() method of the plot renderer of the chart. All you need to do is adding one value to this second series in the point you want the shape to appear.
You can use squares, circles, rounded rectangle and some more. In fact any java.awt.Shape object is valid.