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).
Frederick was more common in the past and has since settled at a lower, steady level. All-time it's been given to 354,782 U.S. babies, the 763rd most common name by sound today, peaking at 6,691 in 1952, with 879 in 2025.
By sound, Frederick also includes the spellings Fredrick, Frederic, Fredric.