  We propose MetroSets, a new, flexible online tool for visualizing set systems
using the metro map metaphor. We model a given set system as a hypergraph
$\mathcal{H} = (V, \mathcal{S})$, consisting of a set $V$ of vertices and a set
$\mathcal{S}$, which contains subsets of $V$ called hyperedges. Our system then
computes a metro map representation of $\mathcal{H}$, where each hyperedge $E$
in $\mathcal{S}$ corresponds to a metro line and each vertex corresponds to a
metro station. Vertices that appear in two or more hyperedges are drawn as
interchanges in the metro map, connecting the different sets. MetroSets is
based on a modular 4-step pipeline which constructs and optimizes a path-based
hypergraph support, which is then drawn and schematized using metro map layout
algorithms. We propose and implement multiple algorithms for each step of the
MetroSet pipeline and provide a functional prototype with easy-to-use preset
configurations. Furthermore, using several real-world datasets, we perform an
extensive quantitative evaluation of the impact of different pipeline stages on
desirable properties of the generated maps, such as octolinearity,
monotonicity, and edge uniformity.
