  Timelines are common visualizations to represent and manipulate temporal
data, from historical events storytelling to animation authoring. However,
timeline visualizations rarely consider spatio-temporal 3D data (e.g. mesh or
volumetric models) directly, which are typically explored using 3D visualizers
only displaying one time-step at a time. In this paper, leveraging the
increased workspace and 3D interaction capabilities of virtual reality, we
propose to use timelines for the visualization of 3D temporal data to support
exploration and analysis. First, we propose a timeline design space for 3D
temporal data extending the timeline design space proposed by Brehmer et al.
The proposed design space adapts the scale, layout and representation
dimensions to account for the depth dimension and how 3D temporal data can be
partitioned and structured. In our approach, an additional dimension is
introduced, the support, which further characterizes the 3D dimension of the
visualization. To complement the design space and the interaction capabilities
of VR systems, we discuss the interaction methods required for the efficient
visualization of 3D timelines. Then, to evaluate the benefits of 3D timelines,
we conducted a formal evaluation with two main objectives: comparing the
proposed visualization with a traditional visualization method; exploring how
users interact with different 3D timeline designs. Our results showed that
time-related tasks can be achieved more comfortably using timelines, and more
efficiently for specific tasks requiring the analysis of the surrounding
temporal context. Though the comparison between the different timeline designs
were inconclusive, participants reported a clear preference towards the
timeline design that did not occupy the vertical space. Finally, we illustrate
the use of the 3D timelines to a real use-case on the analysis of biological 3D
temporal datasets.
