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).
Cindy was more common in the past and has since settled at a lower, steady level. All-time it's been given to 272,102 U.S. babies, the 2,171st most common name by sound today, peaking at 20,995 in 1957, with 110 in 2025.
By sound, Cindy also includes the spellings Cindi, Cyndi, Sindy.