fig2

Figure 2. The timeline visualization mapping of the co-citation network of cited references. (Note: The ring structure reflects the citation history of the literature, where green rings represent earlier years, and red rings indicate more recent years, which can be identified based on the legend at the bottom left of Figure 2. The thickness of each ring is proportional to the number of citations in that year, and the overall radius of a node corresponds to its total citation frequency. Moreover, some nodes are highlighted with purple rings, indicating that these nodes have high betweenness centrality (≥ 0.1), reflecting their importance in bridging different clusters^[51]. The same annotation applies to the image below).