details about lm()

Dear all,
I'd like to get a linear regression of some data, and impose that the line
goes through a given point P. I've tried to use the lm() method in the
package "stats", but I wasn't able to specify the coordinates of the point
P. Maybe I should use another method?
I also have another question: How does lm() choose the point through which
the output straight line goes in order to compute the values of its slope
and intercept?

I would be very grateful if anyone could help me.
Domenico

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Domenico Cozzetto
Biocomputing group
Department of Biochemical Sciences
"A. Rossi Fanelli"
University of Rome "La Sapienza"
P.le Aldo Moro, 5 - 00185 Rome
Tel: +39 06 49690276
Fax: +39 06 4400062
URL: http://cassandra.bio.uniroma1.it/~cozzetto/