There are various versions of this graph floating around, most of which start in 2005 with the publication of the first next-generation (multiplexed) sequencing methods by pyrosequencing (454) and sequencing by ligation (George Church) for complete bacterial genomes. Costs came down very quickly after commercial release in 2006 of 454, ABI Solid (SbL), and Solexa (SbS-sequencing by synthesis). This graph in particular is referring to the cost of sequencing a human genome, which was first done on James Watson using these next generation technologies by 454 in late 2007 if I remember correctly.