Graphs gone crazy

Hi all

Hoping somebody has seen this one before. All of a sudden, all my graphs have gone completely crazy.

The rrd files look good, and contain sane looking data. The curr/min/max summary looks fine, but the graphs themselves are either showing a wash of colour, corresponding to the first defined line, or not showing at all. Just get the link and the magnifying glass. (See attached.)

There appear to be no errors in the web server logs, and none in any of the xymon logs either. :-(

Any assistance appreciated.

Regards Vernon