firstly just a quick note to say I'm responding from memory and iPad so don't have Desktop open to verify..
There's a couple of ways you might approach this
1) If you just right click on the view you have an option to label/annotate an area or a mark or a point, so if your view is quite static, you could just add this annotation ...I think if you choose point, you'll get a text box to write in and a line to associate the text with the point which you can just drag around
2) If youre using dual axis which you say you are, then on the marks card that is associated to your aggregate line, you can choose to label the line ends (click the label button on the marks card of the aggregate line and there are options at the bottom). in v 10 you have the option to just label the latest mark too. You can enter whatever text you want in the text box area of the label dialog screen
hope this helps. If none of the above makes sense, then let me know and I'll post an example with screen shots.