Each philosopher is a node in the network and the lines between them (or edges in the terminology of graph theory) represents lines of influence. The node and text are sized according to the number of connections (both in and out). The algorithm that visualises the graph also tends to put the better connected nodes in the centre of the diagram so we see the most influential philosophers, in large text, clustered in the centre. It all seems about right with the major figures in the western philosophical tradition taking the centre stage. Connections are visualized using gephi (http://http://gephi.org/).