Hi Ben

Normally logging provides you a bit of clue about the reason behind the abnormal behavior or utilisation etc., MRTG graphs only gives you the trends of usage/utilisation of recent past to verify whether all in control..

So logging comes in handy during troubleshooting process. Refer the MRTG graphs for identifying the abnormal behavior/usage and use the logs for clues to find out the reason for the same..

regds