[R] how to overlap plots

[szhan@uoguelph.ca]

Dear R experts:
Can you help me to overlap a histogram and theoretical density curve of poison 
distribution? for example data like this:
The numbers of sanils found in each of 100 sampling quadrats in an area were 
as follows:
number of snails, r 0 1  2 3 4 5 8 15
f=frequency of r   69 18 7 2 1 1 1 1
apparently this is not Poison but near to Poison, How to overlap the 
theoretical desity curve to the histgram of this data?
Thanks in advance!
Josh