On Minimizing Crossings in Storyline Visualizations

In a storyline visualization, we visualize a collection of interacting characters e.g., in a movie, play, etc. by x-monotone curves that converge for each interaction, and diverge otherwise. Given a storyline with n characters, we show tight lower and upper bounds on the number of crossings required in any storyline visualization for a restricted case. In particular, we show that if 1 each meeting consists of exactly two characters and 2 the meetings can be modeled as a tree, then we can always find a storyline visualization with $$On\log n$$ crossings. Furthermore, we show that there exist storylines in this restricted case that require $$\varOmega n\log n$$ crossings. Lastly, we show that, in the general case, minimizing the number of crossings in a storyline visualization is fixed-parameter tractable, when parameterized on the number of characters k. Our algorithm runs in time $$Ok!^2k\log k + k!^2m$$, where m is the number of meetings.