@openscience @jburnmurdoch I've been searching for the data source. The graph seems to rely on data before 1990 but I haven't yet found any source which goes back that far.

The source of the graph appears to be here: https://www.ft.com/content/b2154c20-c9d0-4209-9a47-95d114d31f2b But that doesn't give further details.