adding notes and nodes along a line graph like google analytics, adding pins to map locations, establishing a reference point with additional information. So: A little thing that indicates, and clicking gives you a little more data, which may well NOT be in the little icon, hence it's not just an Icon (which see) and is a different item. ALSO see the eReader project where the highlight can be selected and show more info (not a Tooltip, though, as that is limited, read only, etc.)

This is not a tooltip. Deliberate action needed to reveal the additional info, or always there. tooltips are transient, initiated by hover or automatically presented when the system determines the user needs help -- such as first time, or a change in the system since last visit (put these notes over there when done here). The info in a tooltip is also a helpful sort of label or content add on. It is not content itself which it should be in an annotation.

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