http://i.imgur.com/rKwyy3J.png

according to this chart players can take actions before choosing a plot card.

this is bran http://www.cardgamedb.com/forums/uploads/1281901163/med_gallery_11_134572.jpg

is it legal to trigger bran's ability before the choosing of the plots since there is an action window? if not, what kind of actions are legal during this particular action window?

Yep, you can use Bran before plots are chosen, or any Any phase: or Plot: actions.

Be aware that if you do trigger Bran before choosing/revealing plots in the plot phase, the one you reveal with Bran will be pretty short-lived. Nice way to get two plot effects in one phase, though.