[cairo] line width question

[Carlo]

Hi to all,
I have a question about the line width in Cairo. As defined in the API 
manual, when I specify a line width : "The line width value specifies 
the diameter of a pen that is circular in user space". So when I draw a 
line with a width > 2, the start point P(x,y) is located in the middle 
of the line. For my application purpose, i want to set the start point 
of the line as the left edge corner, how can I do it? I can't figure a 
solution for my problem, the only way I thought is to draw the lines 
with cairo_rectangle, but I can't manage dashed line .

thank you
Carlo Ferraresi