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).
Abby has been slowly declining from its former popularity. All-time it's been given to 102,136 U.S. babies, the 955th most common name by sound today, peaking at 3,210 in 2003, with 525 in 2025.
By sound, Abby also includes the spellings Abbie, Abbey.