The graph shows the changes in the h-index of American Academy of Neurology and its corresponding percentile for the sake of comparison with the entire literature. H-index is a common scientometric index, which is equal to h if the journal has published at least h papers having at least h citations.
This is not the historical change in the h-index. Instead, it calculates the h-index by articles before a given year, not the citations before that year.
