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).
Barbara was more common in the past and has since settled at a lower, steady level. All-time it's been given to 1,448,589 U.S. babies, the 1,479th most common name by sound today, peaking at 49,013 in 1947, with 264 in 2025.
By sound, Barbara also includes the spellings Barbra.