  The visualization of hierarchically structured data over time is an ongoing
challenge and several approaches exist trying to solve it. Techniques such as
animated or juxtaposed tree visualizations are not capable of providing a good
overview of the time series and lack expressiveness in conveying changes over
time. Nested streamgraphs provide a better understanding of the data evolution,
but lack the clear outline of hierarchical structures at a given timestep.
Furthermore, these approaches are often limited to static hierarchies or
exclude complex hierarchical changes in the data, limiting their use cases. We
propose a novel visual metaphor capable of providing a static overview of all
hierarchical changes over time, as well as clearly outlining the hierarchical
structure at each individual time step. Our method allows for smooth
transitions between tree maps and nested streamgraphs, enabling the exploration
of the trade-off between dynamic behavior and hierarchical structure. As our
technique handles topological changes of all types, it is suitable for a wide
range of applications. We demonstrate the utility of our method on several use
cases, evaluate it with a user study, and provide its full source code.
