We recently rolled out some improvements to our alerting feature set. Previously, Librato would create an annotation event every time an alert triggered a service notification (e.g. email, Slack, Campfire). Now, we create one annotation per alert incident (when an alert moves into the triggered state).
In addition, once the alert is resolved (moves out of the triggered state), Librato will update the end_time of the annotation event, resulting in a clearly defined alert incident in your annotations. These changes make it much easier to correlate a specific alert with related trends in your instruments and dashboards.