  A storyline visualization is a layout that represents the temporal dynamics
of social interactions along time by the convergence of chronological lines.
Among the criteria oriented at improving aesthetics and legibility of a
representation of this type, a small number of line crossings is the hardest to
achieve. We model the crossing minimization in the storyline visualization
problem as a multi-layer crossing minimization problem with tree constraints.
Our algorithm can compute a layout with the minimum number of crossings of the
chronological lines. Computational results demonstrate that it can solve
instances with more than 100 interactions and with more than 100 chronological
lines to optimality.
