How to read the band →
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).
Fiona
Fiona is now well below its former popularity. All-time it's been given to 33,964 U.S. babies, the 805th most common name by sound today, peaking at 1,683 in 2017, with 654 in 2025.
Fiona has no common alternate spellings by sound.
Rising
Source: U.S. Social Security Administration, “National data” baby-names files (1880–2025).