If anything I'd say that graph goes against your intended purpose. While I don't place much faith in the Steam Hardware Survey, there's at least a trend there from Feb 2011 to July 2012 that can be followed. It looks like AMD might have lost a few percentage points (from 38-39% to 35-36%) due to the increase in Intel and other GPU products (and is that better detection or an actual market shift?). However, NVIDIA seems to have gone from a 58-60% share down to 46%.
In general I think the market is getting more dynamic due to the increased competition, so things are bound to get shaken up.
I don't really agree. It's a good cross section of gaming, it *will* have the mix of both discrete and igpu in it, but only those which people game with. The loss for nvidia and amd in the past few months all seem to have come at the expense of intel, which we know are 100% igpu scores. However, I suspect that part of that is because of the way virtu reports which gpu is being used (correct me if I'm wrong).
Once we strip out the intel data we have nvidia (discrete, used for gaming) and amd (both discrete and igpu, but only used for gaming). It shows a few things. There are more nvidia cards surveyed than both ingtegrated and discrete amd gpus. Also, AMD is losing share, suggesting that if their igpu strategy is working at all, it is only at the expense of their own discrete parts.
You can discount the steam hardware survey all you want, but it is a very good indicator for the exact market thay nvidia targets (those that game). If you doubt the sample size, I just have to give you this link.... http://store.steampowered.com/stats/ That's concurrent, not total.