page 99 and Fig. 4.1: co-citation

[giaruffo]

I could be wrong, but co-citing papers (connected with dashed blue links to the cited paper) should be both on the left of the target paper, if links points always backward in time.

[filmenczer]

Co-citation means two papers are both cited by one or more citing papers. In the figure, the citing papers are on the right, the cited papers are on the left. Co-cited papers (having a common citing paper) are connected by undirected co-citation links (dashed blue edges).