Right now the 'Report was generated on ...' message that is displayed in the report documentation is only displayed in the table view. It should be displayed on graph views as well.
There are two ways I can think of to add this message:
0) Modify GenerateGraphHTML to create a GenerateGraphData instance directly, instead of re-routing through Piwik_FrontController (see GenerateGraphHTML::getGraphData). The message can be retrieved via GenerateGraphData's viewProperties.