  We present a novel platform for the interactive visualization of very large
graphs. The platform enables the user to interact with the visualized graph in
a way that is very similar to the exploration of maps at multiple levels. Our
approach involves an offline preprocessing phase that builds the layout of the
graph by assigning coordinates to its nodes with respect to a Euclidean plane.
The respective points are indexed with a spatial data structure, i.e., an
R-tree, and stored in a database. Multiple abstraction layers of the graph
based on various criteria are also created offline, and they are indexed
similarly so that the user can explore the dataset at different levels of
granularity, depending on her particular needs. Then, our system translates
user operations into simple and very efficient spatial operations (i.e., window
queries) in the backend. This technique allows for a fine-grained access to
very large graphs with extremely low latency and memory requirements and
without compromising the functionality of the tool. Our web-based prototype
supports three main operations: (1) interactive navigation, (2) multi-level
exploration, and (3) keyword search on the graph metadata.
