The annual rain (and monthly) comes from your console, WeatherCat records it and allows you to plot it but it doesn't calculate it - so if it's 0 in the data editor, that'd explain the drop you see on the graph.
[Edit: If it has dropped to 0, you can edit it back up to where it should be with a text editor - see the manual for details with regard to editing the data - basically quit WeatherCat, open the relevant data file in a decent text editor and edit the annual rain entries that are 0 back to where they should be].
Cheers,
Stu.