The following links to some scripts I wrote to capture sata from my currentcost enerygy meter and log it to a database, then plot it (using flotr).
Home energy monitoring scripts
See it in action here:
Home energy usage
The unit outputs and XML dump every 6 seconds using TTL through its RJ45 socket. I capture this, parse it into MySQL, then on request plot the data over specified time periods.

To plug this into a PC, the TTL (5v/0v) levels must be converted to RS232 levels (+12v/-12v). This can be done using a very simple circuit and a MAX232 chip, or even easier one can be bought on ebay from the people who make the monitoring units themselves:
Currentcost Data Cable