Fix index page chart when lots of data

[jantman]

With lots of historical data (I currently have 5 years of daily balances), the "Account Balances" chart on the index page is taking an extremely long time to load and is almost illegible. This chart should be changed to either limit the amount of historical data (make that configurable) or else just provide a summary of the data (i.e. only show every N data points).

It's possible that if we do #215 and use a more advanced chart library, we could default to only showing the last N data points (or days/months/years) and then allow the user to "zoom out" in the UI to show more data.