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. The algorithm that visualises the graph also tends to put the better connected nodes in the centre of the diagram so we get the most influential philosophers, in large text, clustered in the centre.
Without Christianity (from Jesus Christ to Church) much of figures placed in right half of this image can’t make sense though. How such giant and obvious mistake is possible … ?