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).
Marlo has been climbing in recent years. All-time it's been given to 21,507 U.S. babies, the 709th most common name by sound today, peaking at 1,074 in 2025, with 1,074 in 2025.
By sound, Marlo also includes the spellings Marlowe, Marlow.
Source: U.S. Social Security Administration, “National data” baby-names files (1880–2025).