Since the antibody reaction is on a log scale I would be more confident after 28 days than after 14 days based on that graph. But the n for the different time ranges (days after vaccine) are much too small to be sure, of course (and I am not a medical expert).
Maybe figure 1 of this article: Antibody Responses in Seropositive Persons after a Single Dose of SARS-CoV-2 mRNA Vaccine could be helpful. I think relevant are the blue dots (for persons seronegative before the first vaccine).
Since the antibody reaction is on a log scale I would be more confident after 28 days than after 14 days based on that graph. But the n for the different time ranges (days after vaccine) are much too small to be sure, of course (and I am not a medical expert).