in my understanding nadis are subtle channels like veins on nerves where prana flows while chakras are vibrating centres in those. Main nadi is still 'shushumna' which runs inside the spine and that's a flow channel and chakras are centres or 'stations' of those. On this picture for egzample we can see that both chakras and nadis extend beyond body. I would encourage further research into the subject in yoga-sastra as i'm not an expert.