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).
Julia was more common in the past and has since settled at a lower, steady level. All-time it's been given to 481,776 U.S. babies, the 298th most common name by sound today, peaking at 8,915 in 2001, with 2,375 in 2025.
By sound, Julia also includes the spellings Giulia.