Add options to visualize sub-graphs of a large graph (factsheet as a graph).

[adelmemariani]

The RDF-based factsheets have the potential to become very large. Therefore, there should be some possibility in the tool for visualizing the sub-graphs. This means, by clicking a node in the "playground" mode of the "oekg-builder", the nodes and the edges that are directly connected to the clicked node should be highlighted (and the rest of the graph should be faded). In other words, the subgraph represents the immediate neighborhood of the clicked node. Also, the subgraphs should be in a separate part of the UI (i.e., a pop-up window).