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David François Huynh while at MIT and subsequent developers have created a suite of open source visualization tools called SIMILE including a timeline widget. The timeline is clear, and easy to navigate. On their site, example time lines like this one of JFK's assassination show the data at multiple time scales. individual events are labeled for quick review, and you can see both single point events and events with a duration. One uncertainty is that the site states they support zoom functionality but forum posts indicate there may be some difficulties in the implementation.