The UI graphs are lacking in ability to distinguish particular events that sometimes need to behighlighted due to some peculiarities in the data. The goalis to allow for users and analysts alike to have a visible pointer or reminderthat something is fishy in the data or for example that a traffic spike was due to some bot traffic from bogustown.
Ideally, users should be able to pick a section of a graph, add a marker/pointer (like the old calendar events) to indicate a data issue or comment. Those comments or annotations though need to become part of the data set. i.e. the date associated with the annotation need to become tied to the event, so if you decide to trend the data in Analytics, DW or Ad hoc, an annotation field is displayed as well. As if you are allowing users to add an additional column in the field tied to a date. It would also be ideal in ranked reports where the date in included in the date range to also load a field highlighting the annotation.
However, the ideal solution when the issue is one of erroneous traffic would be to allow clients to work with client care so data reprocessing can be performed and filter out that bot traffic or bogus traffic so it's retroactive (yes there is a bot and IP address exclusion solutions or even vista rules but nothing is retroactive - i.e. gimme some transformation like in Workbench). This allows to no longer have to worry about setting a reminder mainly when you look back at historical data or when users -unfamiliar with the dataset- pull reports.