Solid line = floor (spellings that unambiguously belong to the group). Shaded band up to the dashed edge = ceiling (the most it could reach if every ambiguous spelling counted here).
Susan was more common in the past and has since settled at a lower, steady level. All-time it's been given to 1,135,103 U.S. babies, the 1,680th most common name by sound today, peaking at 47,967 in 1955, with 222 in 2025.
By sound, Susan also includes the spellings Suzan.