  The Eclipse Layout Kernel (ELK) is a collection of graph drawing algorithms
that supports compound graph layout and ports as explicit anchor points of
edges. It is available as open-source library under an EPL license. Since its
beginning, ELK has served both as a research vehicle for graph drawing
algorithms, and as a practical tool for solving real-world problems. ELK and
its transpiled JavaScript cousin elkjs are now included in numerous academic
and commercial projects.
  Most of the algorithms realized in ELK are described in a series of
publications. In this paper, the technical description concentrates on the key
features of the flag-ship algorithm ELK Layered, the algorithm architecture,
and usage. However, the main purpose of this paper is to give the broader view
that is typically left unpublished. Specifically, we review its history, give a
brief overview of technical papers, discuss lessons learned over the past
fifteen years, and present example usages. Finally, we reflect on potential
threats to open-source graph drawing libraries.
