For this demo, we used semantic data derived from ozone and visibility readings provided by the EPA's Castnet project. This semantic data combines raw data on Castnet readings (ozone and visibility) with corresponding geographic information on the sites of readings, which the raw Castnet data from data.gov lacks. On the provided map of the US, every yellow dot represents a single Casetnet site and dot size corresponds to an average Ozone reading for that site, where larger dots represent larger averages. When a Castnet site is clicked, a small pop-up opens, displaying more information on that site, along with a link. This link takes the user to another page that displays in a timeline all the Ozone and Visibility data available for that site. This timeline uses the Google Visualization API giving users added functionality in how they view the data in the timeline. To allow users to filter through Castnet readings, a faceted browsing interface (from the MIT Simile Exhibit API) is provided.
You can see the demo at http://data-gov.tw.rpi.edu/demo/exhibit/demo-8-castnet.php and read more about this application and its development process at http://data-gov.tw.rpi.edu/wiki/Demo:_Castnet_Ozone_Map