Fig. 3.

Timeline visualization of co-citation references network. Note: A node represents a cited reference. The size of a node depends on its betweenness centrality. For each cluster, nodes are organized by their year of publication on horizontal lines. Nodes with large coloured tree rings are those with high betweenness centrality (external purple tree rings) and burst strength (central red tree rings). The colour of lines indicate the time of links between nodes or between clusters