Notes can be added to statistics and display along with the graph for easy viewing. Anything can be entered on a note, from a change in the way the stat is being calculated to a note about what was occurring within the company that week.
Under the "Enter Values" tab, you can go to either "Stat Group Entry" or "Single Stat Entry" and after selecting a stat, see a column to enter notes on.
After entering your note, click "Save". The note is now visible on the Graphing tab. Values which have a note entered display as a heavy, enlarged dot on the graph. Hover your mouse to view the note: