I received a Efergy Elite energy monitor (sold in Maplin and other retailers) as a Christmas gift few years go. The Elite provides current use, daily and weekly average and all that green CO₂ nonsense. While useful for tracking down unnecessary drains it doesn't give you a true picture of electricity use throughout the day.
Despite getting all this juicy data every 6 seconds the Elite won't share it with you. There is no computer port. So I hacked it.
I soldered a wire on the baseband signal from the radio receiver, into a cheap little PIC MCU for some decoding and at the other end a stream of numbers every 6 seconds. Bung that into a graphing package and you get a chart like that above.
[*] Actually it can only sense current. So you assume that the supply is at a constant 230V rms and you can calculate the power (power = V*I). It's not accurate enough for billing, but certainly good enough for consumer energy monitoring.