I found a strange diagram on Stephen O'Malley (of Sunn0))))'s blog:
It purports to track metal releases - broken into 'full lengths' and 'demos' - between 1980 and 2009. The diagram shows an accelerated increase since around 2000 followed by a precipitous decline in the last two years. It's unclear what counts as 'metal' and indeed what counts as a demo.
In fact the diagram is almost certainly made up. I checked with Stephen O'Malley and he doesn't know who authored the diagram (he got a copy from a friend) and it only seems to appear on one other place on the web (although I didn't search that hard).
So it's all a bit of a mystery. Still, I'd love it if it were possible to produce a diagram of this kind accurately, or even a reasonable estimate.
It reminds me of the criticism of over-production in metal in the incomparable fanzine Isten 100. Might the bubble have burst now?
UPDATE: An anonymous commentator pointed out that the diagram comes from Cosmo Lee's Invisible Oranges blog, compiled from data from Metal Archives . This is embarassing - I do read that blog but I've got a bit behind in my RSS reader recently so I didn't see the post when it came out last week.