This plugin adds "blocks" and "blocked by" fields to each ticket,
enabling you to express dependencies between tickets. It also
provides a graphviz-based dependency-graph feature for those tickets
having dependencies specified, allowing you to visually understand
the dependency tree.

The dependency graph is viewable by clicking 'depgraph' in the
context (in the upper right corner) menu when viewing a ticket that
blocks or is blocked by another ticket.

WWW: http://trac-hacks.org/wiki/MasterTicketsPlugin