Redmine

What is the best way to display a group of issues under one heading e.g Absence (parent task) and subtasks are all of the items under this heading or do i need to group the issues together in a different way so they can displayed in the gantt chart - ideally i would like a tree structure in the gantt where you can show high level topics and then open it up if needed - can someone advise the nest approach or if i need a plug in to achieve this? Many Thanks