12. The method of claim 11 , wherein: receiving a selected topic among the set of topics comprises receiving a selection of a first topic and a second topic; and forming the relationship graph of the documents comprises: retrieving from memory a first pre-calculated relationship graph corresponding to the first topic; retrieving from memory a second pre-calculated relationship graph corresponding to the second topic; and forming the relationship graph based on both the first pre-calculated relationship graph and the second pre-calculated relationship graph.
11. The method of claim 1 , comprising: before receiving the selected topic among the set of topics, pre-calculating and storing in memory a plurality of relationship graphs corresponding to respective selections of each of at least a plurality of topics in the set of topics.