You can add notes to the trend view graph either by creating a new note with original content, or by creating a note from an alert.
Notes are a great way to supplement the data on the profile dashboard. You can add a note about the start of a new campaign, leave yourself a reminder to investigate drops in traffic, or convert alerts to notes so you have a permanent record of when your profile hit certain benchmarks.
To toggle notes on and off, click the Notes bar in the annotations menu. When notes are on, Insight adds a flag to the trend view graph for every note. If there are other annotations for the same day (notes, alerts, and RSS feeds are all annotations), they are combined in a flag.