Tim - some thoughts:
It sounds to me like this should be a website. You upload your polar (etc) files to it, and all charts, calendars, etc are web based. The advantage of this is that users could make their stats public (for some good-natured competition) and could foster a community. Also, users wouldn't have to install an app, and could access their info from anywhere.
Also, if you wanted to move the project towards being commercial, it is easier as a website (google ads), as people just don't shell out cash for utilites of this sort very often anymore.
I'm actually a fan of thick-clients (i.e. a desktop swing-based Java GUI) where applicable, but I don't think that there are any UI elements involved in software like this that justify being a thick client.
As far as Java goes, if you get stuck, log into the ##java channel on
http://freenode.net - I'm often hanging out in there (same nick), but so are a bunch of other Java experts. Even if you do it as a website - I still recomend Java for the server-side stuff, although PHP would also be suitable.
Good luck