Allow large amounts (longer time periods) of data to be graphed efficiently

[jpasqua]

Right now the amount of data loaded into the graph is effectively limited by RAM and the very inefficient memory model of LineChart and related classes. If a user loads more than about 30 days worth of data (like a year or more), the app becomes unusable. There are a number of ways to potentially address this issue. For example, we could find or implement a graphing class that loaded data lazily rather than upfront. We could subsample older data. We display at most 30 days worth of data, but require the user to "page" through 30 day sets of data.