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).
Marvin was more common in the past and has since settled at a lower, steady level. All-time it's been given to 255,807 U.S. babies, the 1,186th most common name by sound today, peaking at 4,484 in 1928, with 385 in 2025.
Marvin has no common alternate spellings by sound.