Fig. 6

A Top 10 keywords clusters by LLR from 1973 to 2022. Upper right picture is the individual part of the 1,2,3,4,6 composition, and lower right table shows the dominating keywords of each cluster. B Timezone map of keywords (Frequency≥5) between 2003 and 2022. Each node in the year column means the keywords first occurrence in certain year, the color rings means it occurs in the following years  (from blue to red as in the left bar). C Timeline map of title word between 2003 and 2022. Circles appeared in each timeline means first research beginning time. The bigger the circle, the more co-occurrence. Colors of rings means appearing in different year (from blue to red as in the left bar). The right table shows detail message of each cluster. D Keywords with the strongest citation bursts. Red bar means time of keywords bursting, and blue bar means the occurrence time