How is Network Deficit Calculated? I've seen this graph pop up around the interwebs, but never with an explicit explanation:

http://blockchain.info/charts/network-deficit

My question is how is this metric generated? Is it a comparison of the power and hardware costs versus returns?
This was described as showing the difference between transaction fees and the bitcoin miners income from block rewards.

So if a Block has 25 BTC generated and 1 BTC of fees, the network deficit would be 24 BTC.  The amount in USDs is calculated at the spot rate at the time.