It isn't natively supported sorry.
One way of working around this is to allow server collaboration objects. A user can then create a text object with transparent background where he adds his notes and then position this on top of the graph.
Not a very pretty solution but the only way I see this can be achieved. I would recommend instead to put the practice in place that if a point of interest is found, the selection is bookmarked and the user send the bookmark together with an explanation in the email about what he found.