What would be the best way to make "unlimited" charts in ExtJS 4?

With unlimited, I mean that the data is stored server-side. The client requests only the current time range (or some range around that) at a the current zoom factor (LOD). A kind of paging...

The obvious applications would be financial data, bandwidth graphs and other measurements over time. I have huge amounts of time series data I'd like to visualize, loading them all at once would not be possible.

Currently, I have a "From" and "To" date selector above the graph, with which the user can set the date range. This is not very user-friendly, it'd be nice if the graph itself would request data if it is currently not available (like Google Finance).