Your comments

I'm having problems getting the unit I received from FooGadgets to work. I'll be able to look at it further next week. If I'm unable to get the unit working then I'll have to send it back for a replacement. It's difficult to add support for the device when I can not test it properly myself.
You need to use telldus live since that is where Tellmon gets the data from. For the duo you need to download the windows software from Telldus (can't remember what it's called).  It has the option to send your data to telldus live. Check telldus' website for how to do that. Then your computer that the duo is connected to needs to be running and have the windows software running.
Sum does not give any meaning for rain sensors. Summing rain rate or rain total (which is already summed) makes no sense. 
The same problem was for the wind sensors as well. Wind direction does not seem to work though. Perhaps that's not supported.
Can you have a try now? The channel names reported via the push API (that sends the data) and Telldus Live API for rain sensors did not match. One uses 'rrate' and the other 'rain rate'. One uses 'rtotal' and the other 'rain total'. Now I support them both.
Tellmon only logs and displays what ever data is received by Telldus Live. It therefore supports those devices that functions on Telldus Live. Supported sensors sends the data to your tellstick device which sends them to Telldus which sends it to Tellmon.

If the Oregon sensors works with Telldus Live it should work with Tellmon as well. If not there is really nothing I can do. It should start working if and when Telldus starts supporting it.
I'm away on holiday and will not be able to look at it until I'm back.
Hi

I don't have a rain sensor myself so I'm having difficulty testing this, but I'll have a look at your data and see what I find. My guess is that the rain sensors only send data when it's raining (probably each time the bucket is full and tips). If no data is logged (no rain) during the time you look (day, week, month) the graph has nothing to plot and I expect that is why you get the 1970 timestamp (a time value of 0 in javascript equals 1/1/1970 00:00:00)
Have you tried? I just bought a Nexa magnetic switch and it logs every time my front door is opened and closed. I would expect the doorbell to work similarly. You will need to add the doorbell to your telldus live devices using the (learning command).
I'm just a single person and I already have a day job. So Tellmon is basically my hobby project. Speed of development is dependent on how much time I can devote to the project. That is dependent on what else needs to be done around the house and the activity level of stuff with my spouse and children.

Right now I'm working on that Telldus Live call events. A way for you to trigger devices and alerts based on sensor or device activity. I'm hoping it will be released some time during summer. But I'm soon going on a three weeks vacation and all development is going to stop then.

Regards
Pål