Wish I'd thought to ask this question myself. The paper doesn't have the data to answer this, but with a little work one can search PubMed for retractions. Interestingly, I get 2,418 hits while the paper mentions only 2,047.

I grabbed the 441 retraction records for 2011 (in MEDLINE format for easier processing) and slapped together a script to extract the references. There were 458 (some retractions are for multiple publications) and my script pulled 415 publication years from them. Some papers had no year because they were referenced only by DOI and the DOI didn't include a year; some papers had two publication years because they had a formal publication date and a DOI with a year (presumably these papers are the ones that appear online before getting a formal volume number & page allocation).

A tally of the original publication years for those 415 retractions:

• 1998: 2
• 1999: 8
• 2000: 7
• 2001: 13
• 2002: 20
• 2003: 11
• 2004: 13
• 2005: 33
• 2006: 33
• 2007: 31
• 2008: 32
• 2009: 66
• 2010: 109
• 2011: 37

Looks like really old papers aren't being retracted, which surprises me! I would've expected at least a handful from the 1980s, but last year's retractions stop dead at 1997. Somehow I doubt the shitty pre-1998 papers have all already been retracted.

[Edited to fix list and 2003 figure.]