Tasks (that I'm aware of):

Found the papers for CLEF-IP 2012, and TREC 2011 Chemical IR (the two
that are pertinent).

[ CLEF Summary: http://www.ifs.tuwien.ac.at/~clef-ip/tasks.shtml ]
CLEF-IP 2009
CLEF-IP 2010
CLEF-IP 2011
CLEF-IP 2012 *chem diagram recognition; not sure about others
CLEF-IP 2013 [ only flowcharts for extraction ]

TREC 2009 Chemical IR (first year) - no image rec.
[[ Does not seem to have occurred in 2010 ]]
TREC 2011 Chemical IR *chem image2structure task (Birmingham won this)