Gaia Data Release 1: The archive visualisation service
  Context: The first Gaia data release (DR1) delivered a catalogue of
astrometry and photometry for over a billion astronomical sources. Within the
panoply of methods used for data exploration, visualisation is often the
starting point and even the guiding reference for scientific thought. However,
this is a volume of data that cannot be efficiently explored using traditional
tools, techniques, and habits.
  Aims: We aim to provide a global visual exploration service for the Gaia
archive, something that is not possible out of the box for most people. The
service has two main goals. The first is to provide a software platform for
interactive visual exploration of the archive contents, using common personal
computers and mobile devices available to most users. The second aim is to
produce intelligible and appealing visual representations of the enormous
information content of the archive.
  Methods: The interactive exploration service follows a client-server design.
The server runs close to the data, at the archive, and is responsible for
hiding as far as possible the complexity and volume of the Gaia data from the
client. This is achieved by serving visual detail on demand. Levels of detail
are pre-computed using data aggregation and subsampling techniques. For DR1,
the client is a web application that provides an interactive multi-panel
visualisation workspace as well as a graphical user interface.
  Results: The Gaia archive Visualisation Service offers a web-based
multi-panel interactive visualisation desktop in a browser tab. It currently
provides highly configurable 1D histograms and 2D scatter plots of Gaia DR1 and
the Tycho-Gaia Astrometric Solution (TGAS) with linked views. An innovative
feature is the creation of ADQL queries from visually defined regions in plots.
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