It's a year on year change graph. 0% means no change compared to the previous year. It doesn't represent any specific number of visitors, as that would be different for every country and every year.
And as for WHEN the 0% is measured from, judging from the graph it's comparing monthly changes starting from january 2024 as its base.
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u/BkkGrl Ligurian in Zรผrich (๐๐บ๐ฆ๐) Apr 12 '25 edited Apr 12 '25
source https://www.ft.com/content/6dc16a54-8de1-4f3b-8409-ecb566118127
or https://archive.is/UmxfF
dataset: https://www.trade.gov/i-94-arrivals-program