[NTG-context] Positioning graphics at positions on a page

Hi, Measuring from the top left hand corner of US letter paper, I'd like to position four identical metapost graphics, centered at (2in,2in), (6in,4in), (2in,6in) and (6in,8in). The first entry in each pair is the amount I'd like to move to the right from the top left corner of the page. The second entry is the amount I'd like to move down from the top left corner of the page. Any ideas?