I'd also question the validity of it. Just because you 'paid' a spread of say 3 pips on GbpUsd for a trade, does not mean that your broker made 3 pips. He gets his price on the open market ( providing he's a genuine broker accessing the open market ) and then potentially added his cut on top of this.
There are three main setups with regards to brokers.
Fixed spread, so he makes a varying 'commission' based on what the spread was at the time he put the trade through. He could also conceivably make a loss on some trades. Although I'm sure this is rare.
Variable spread, commission based model. In theory your broker shouldn't be making any money on the spread as he should be giving you the raw feed price. This is the best model in my opinion as your broker has no interest in your trade providing he gets his commission. Brokers such as Dukascopy and Jade use this model.
Variable spread, non commission based. Your broker gets the current price from the open market and adds on a number of pips in order to scalp his commission off the trade. I think in general most brokers have a fixed number of pips for this, although I'm not 100% sure.
There's then obviously the subscription based model of Collective FX.
So, I can understand the interest in seeing spread compared to trade but if you want it purely to see what the broker is making off your trades then you aren't going to get it from this metric.
11:15, restate my assumptions: 1. Mathematics is the language of nature. 2. Everything around us can be represented and understood through numbers. 3. If you graph these numbers, patterns emerge. Therefore: There are patterns everywhere in nature.