I wrote a small windows app which can display the monthly/daily bandwidth information as line chart. I wrote the application for my personal use but it is open source (hosted in github https://github.com/tparvi/tomatoui).
If you could would you test it and report back in this thread whether it works. I would like to know what router and which version of the tomato firmware you are using. I'm using the app against WNR3500L (v1.28 or the tomato firmware). Since I don't have any other routers or versions of tomato firmware installed I have no idea whether it works. You can report bugs here or use the issues page at github (https://github.com/tparvi/tomatoui/issues)
System requirements: Windows, .NET Framework 4
You can download it here
https://github.com/tparvi/tomatoui/archives/master
https://github.com/downloads/tparvi/tomatoui/TomatoUi_v1.0_beta.zip
For more information you can see the wiki
https://github.com/tparvi/tomatoui/wiki
Posted this on reddit. Seems to work with at least other router/tomato firmware. I'm pretty confident it will work with other tomato flavors as long as people don't change the way bandwidth information is stored and displayed in the web page.
http://www.reddit.com/r/hardware/comments/ko36w/if_your_router_is_using_...