My problem is rather simple actually. We currently track events in a local database very similarly to the events we would send to the Mixpanel. These events have been analyzed only superficially until now. If I wrote the code to analyze and plot the data, it would work on new data also and would make Mixpanel somewhat redundant (dealing with updates is a non-issue, it's the logic that's difficult, not dealing with more data).

However, I don't want to reinvent the wheel, or to invest a lot of time in something that other apps like Mixpanel, NuConomy already do very well. I suspect Mixpanel uses the timestamps of the events for billing and if so they might require some non-trivial changes to their business logic to allow historical data. I hope they will, someday. Until then I might settle on sending them only new events and using R on the historical data.