The Supervisor Reports API limits the report range to a 24 hour period, but the API can be automated to run at an interval. The resulting CSV export file can be parsed and stored in a database, or simply archived.
https://servername.lithium.com/reports/csv_long/rawPost?startTime=1396224000000&endTime=1396396800000
The values for startTime and endTime are in milliseconds (Epoch). Online converters such as http://www.epochconverter.com/ provide a 'human date to timestamp' conversion option.
The time span between the start and end times cannot exceed 24 hours. In the example above, the range is 7/28/2013 00:00:00 to 7/29/2013 00:00:00. The tzOffsetMs parameter specifies the time zone. "-18000000" is used in the example to indicate Central Daylight Saving Time.