Mercy
I didn't know either.
I googled "legend for a lab report?"
https://www.google.ca/search?sourceid=navclient&aq=&oq=a+legend+for+a+lab+report%3f&ie=UTF-8&rlz=1T4GGNI_enCA530CA530&q=legend+in+a+lab+report+&gs_l=hp..0.0i22i30.0.0.0.11864...........0.znElakmxh8M
See the section called " TABLES"
http://www.bio.davidson.edu/molecular/Protocols/papers.html