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).
Marcos
Marcos has stayed fairly steady in American naming for generations. All-time it's been given to 53,742 U.S. babies, the 922nd most common name by sound today, peaking at 1,415 in 2001, with 579 in 2025.
Marcos has no common alternate spellings by sound.
Rising
Source: U.S. Social Security Administration, “National data” baby-names files (1880–2025).