It would be great to be able to add annotations to the data on the graphs to comment specific events, add remarks of reminders for yourself and other users.
There are a lot of applications for this feature. E.g. your making various changes in the configuration of your sever one at a time, and you would like to monitor, how each of them affects, for instance, the load on the server. This information could be easily displayed on graphs, with annotations explaining, which change was applied and when.
The screenshot shows, how it's done in google analytics.