  Small multiples are miniature representations of visual information used
generically across many domains. Handling large numbers of small multiples
imposes challenges on many analytic tasks like inspection, comparison,
navigation, or annotation. To address these challenges, we developed a
framework and implemented a library called Piling.js for designing interactive
piling interfaces. Based on the piling metaphor, such interfaces afford
flexible organization, exploration, and comparison of large numbers of small
multiples by interactively aggregating visual objects into piles. Based on a
systematic analysis of previous work, we present a structured design space to
guide the design of visual piling interfaces. To enable designers to
efficiently build their own visual piling interfaces, Piling.js provides a
declarative interface to avoid having to write low-level code and implements
common aspects of the design space. An accompanying GUI additionally supports
the dynamic configuration of the piling interface. We demonstrate the
expressiveness of Piling.js with examples from machine learning,
immunofluorescence microscopy, genomics, and public health.
