Maybe a date formatting problem? I saw something similar in the past due to passing the wrong format string, and Jan 2 was the Saturday of the "last week" of 2020, before the "first week" of 2021?
I think it's more likely that there were one or more manual steps in generating the graph, and someone just kept going with 2020 instead of typing 2021. The author should explain what went wrong there if this reaches their attention.