I now have code which is (in theory) generating a GEXF file with nodes and edges based on the co-occurrence of locations in the data collection, but it's taking hours to run, so I haven't yet managed to complete a successful run and see the resulting data. I've tried to cut down the amount of calculation by limiting the nodes to those which are referred to more than ten times, but it's still taking too long, so I'm trying to debug.