Faheem this has been asked several times in the past couple of months.
check ?curve and the parameter 'add' as in
curve(fc(x), 0, 10, col = "darkblue", add = TRUE) # from the reference manual
If this isn't what you want then search Jonathon Baron's archive at
http://finzi.psych.upenn.edu/search.html for numerous other solutions,
Richard Rowe
Senior Lecturer
Department of Zoology and Tropical Ecology, James Cook University
Townsville, Queensland 4811, Australia
fax (61)7 47 25 1570
phone (61)7 47 81 4851
e-mail: Richard.Rowe at jcu.edu.au
http://www.jcu.edu.au/school/tbiol/zoology/homepage.html