Problem with manual creation of avg/extremes

As my PC is rarely up at around midnight I have to manually create the avg/extreme pages. This causes a problem with the graphs. If you take a look at http://www.broadstairs.org/20041225.gif and compare it with http://www.broadstairs.org/20041224.gif you will see the problem. The one for the 25th was created manually at around 07.40 on the 26th and the one for the 24th was created automatically by WD at 00.05 on the 25th as my PC was still up (since I was dragged to the midnight service by the wife).

I had expected WD to create the files as if it had been run at 00.05hrs.