The GOLL-E Content Language specifies the content of the graph, and any data pertaining the graph. That is the list of vertices contained in the graph, with each vertex detailing the data specific to itself, and a list of edges, with each edge detailing the data specific to itself. The content language is specified in greater detail here. The visualizer plays a key role in this system because it has to support user modification (i.e. dragging vertices to modify the layout).