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).
Robin has been climbing in recent years. All-time it's been given to 413,901 U.S. babies, the 729th most common name by sound today, peaking at 19,623 in 1961, with 941 in 2025.
By sound, Robin also includes the spellings Robyn, Robbin.
Source: U.S. Social Security Administration, “National data” baby-names files (1880–2025).