If you are using PI ProcessBook, you can create a trend and add a PI Point with at least one annotation within the selected time range. The annotation will be shown as an icon on the top of the trend. If you mouse over this icon, you will see its content. Please refer to the following image:
Let me know if this is what you are looking for,
One more thing in addition to Marcos' post: the annotation can be added to the event of the tag, not at the random timestamp. If you right-click on the trend and select Details and Annotation (you should be in the Run mode to see this option), you will see the list of events for this tag which you can annotate directly from PI ProcessBook. You can also annotate an event with PI SMT or programmatically.