i'm not particularly impressed by the graph. The NASDAQ index also had a nice run a few years ago. Then people got all sad and weepy over the dot.com crash.
just as an example of how disingenuous that article is, they imply that Au is still underpriced because it "is still at half the peak set in 1980, after adjusting for inflation". Of course, they fail to mention that 1980 was a bubble and the market crashed in 1982.
There is no shortage of gold as demonstrated by the piles of it sitting around in banks. There is also no lack of capacity to mine it. I'd bet this is a particularly sweet time for mining companies because their equipment costs are not being driven up by competition for manufacturing supply due to high oil, coal, and natural gas prices. In addition, unemployment is high so labor costs are down (it's not like these are highly skilled jobs). I'd be surprised if avg production cost/oz is above $350/oz in the US.
That production is not expanding is probably more a matter of consolidation in the mining industry and their ability to better control the market rather than a diminishing reserve base. However, that doesn't necessarily make Au miners a good investment 'cause I think there profitability is well baked into their prices.