This gadget is used within a Google spreadsheet to create a visual timeline
of dated information. The source spreadsheet must contain the following fields:
title: description of each event
start: the start date of the event
end: the end date of the event (optional)
image: a URL of an image which should show when the event is clicked
link: a URL which will appear as a link in the popup when the event is clicked
description: a descriptin of the event which shows when the event is clicked.