I finally found the Timezone issue which is causing some graphs to move 1 or 2 hours into the future or past.
This problem is caused by the fact that the programming language Java always tries to convert to the local settings of the computer, while the times in the logfiles already are in the right timezone. Java would add 1 or 2 hours depending on the timezone and daylight savings time.
The fix is quite easy but also cumbersome at the same time. I now assume everything is in GMT and also parse the dates differently now.
This means I have to retest all the dataloaders if they work correctly, due to the changes I had problems getting the Sunny Data Control reader to work properly again. I hope to post the fix soon.