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).
Jonathan is down from its peak but still widely used. All-time it's been given to 1,064,539 U.S. babies, the 138th most common name by sound today, peaking at 31,192 in 1990, with 4,606 in 2025.
By sound, Jonathan also includes the spellings Johnathan, Jonathon, Johnathon.
Source: U.S. Social Security Administration, “National data” baby-names files (1880–2025).