There is a community module called fstr that may be useful.

All accounting Reports are centralized one... "account.common.report" acts as an base model for all reports, featuring all basic fields with necessary requirements... and rest all other reports inherits this object and later addition report criteria are added... So for Profit And Loss Report, refer "accounting.report" object..

Path: account/wizard/account_financial_report.py

Tip: Use Ctrl + H, which performs a search on entire addon/openerp folder, if you are using EDI like Eclipse, Aptana Studio, etc