I am having my first bout with weird graphs issues also. Though it is not related to the end of month problem. I have a dropped extra sensor, a strange instant fall in temperature which affected humidity & cloud base, a fluctuation in wind direction & barometer, and the addition of precipitation totals while logging a zero rain rate.

All at the same time.
Forcing a refetch of the logger by changing the data.cat file name produced better results except the dropped extra sensor. However my logger frequency is short so I would loose the majority of March.
A permissions rebuild fixed the dropped sensor and all graphs appear back to normal save a spike on all temp graphs at the moment of returning to the original data.cat file.
None of the wired events appear in the WeatherCat stats, on the Davis Console, nor the Weatherlink for windows side of things (I run VMWare and fetch weekly to maintain it) so the oddities seem to be only with the graphs. Super odd and disappointing.
This has not happened since the systems installment back in Nov/Dec. so I'm stumped as to why it's suddenly happening now. I thought this thing was doing fine. My trust level has dropped significantly.