Roassal maps objects and connections to graphical elements and edges. In additions, values and metrics are represented in visual dimentions (e.g., width, height, intensity of graphical elements). Such mapping is an expressive way to build flexible and rich visualization. This chapter gives an overview of Roassal and its main API. It covers the essential concepts of Roassal, such as the view, elements, shapes, and interactions.
I would try a less technical introduction to combine with this one. How about:
When we're building a visualization, we want the properties of the objects in our domain to be expressed graphically, by shapes, connections and visual dimensions like width, height, intensity of graphical elements. Roassal builds such mappings as an expressive way to build flexible and rich visualizations.