The sentence in question is,
"Hardening timescales are plotted against separation in Fig.2, broken down by hardening mechanism."
It was suggested that I change this to
"hardening mechanisms". I'm pretty sure this is incorrect, and the interwebs seem to agree (although this is a difficult topic to search for). See: http://alt.usage.english.narkive.com/f3XfjY8U/in-a-breakdown-by-singular-or-plural,
http://www.indiana.edu/~wts/pamphlets/proofing_grammar.shtml (has example using singular).
1) Is there a name / description for this type of situation / problem? Are there other constructs it is analogous to?
2) Which is correct, singular or plural? And why (i.e. what is the general rule)?