Saved and Discarded Graphing Data
Metrics graphing data is calculated and saved to the Metric Graphing database. When the Graphing window is opened, it can immediately retrieve from the server historical data for a configured interval, plus the same amount of future data for forecast metrics.
Persistent Graphing Data
Graphs that you created, or are in the process of creating, persist if you intentionally or accidentally log out. When you log in again, these same graphs, including the metrics you selected for graphing, are retained until you deselect the metrics or close the graphing window.
See also, Graphs Saved in User Preferences.