It is possible to add an annotation to the graph to a specific point or to a selected time range.
To add an annotation on specific point:
Click a point on a graph where you want to add the annotation.
Click Add annotation.
Type the annotation in the description field.
Click Save.
When hovering over this point, the annotation is displayed.
To add an annotation on a time range:
Select the time range area while holding the Ctrl/Cmd key.
Type the annotation in the description field.
Click Save.
When hovering over this area, the annotation is displayed.