When viewing a time-series, you often wonder why there is a huge dip, or what caused a spike in your data. This is where Annotations will help - to provide contextual information on what happened at a particular date.
You can access the Annotations page from the Tools menu:
In the Annotations page, enter the event date, title and the details of what happened:
Then click Submit.
The letters on the chart represent annotations on that date. When clicking on the letters, the annotation sidebar will expand.