  External labeling is frequently used for annotating features in graphical
displays and visualizations, such as technical illustrations, anatomical
drawings, or maps, with textual information. Such a labeling connects features
within an illustration by thin leader lines with their labels, which are placed
in the empty space surrounding the image. Over the last twenty years, a large
body of literature in diverse areas of computer science has been published that
investigates many different aspects, models, and algorithms for automatically
placing external labels for a given set of features. This state-of-the-art
report introduces a first unified taxonomy for categorizing the different
results in the literature and then presents a comprehensive survey of the state
of the art, a sketch of the most relevant algorithmic techniques for external
labeling algorithms, as well as a list of open research challenges in this
multidisciplinary research field.
