gtk-text-iter-visible-line-offset (iter)

iter -- a gtk-text-iter instance
char-on-line -- an integer with a character offset

Accessor of the character offset relative to the start of the current line of the iterator.

The function gtk-text-iter-visible-line-offset returns the offset in characters from the start of the line to the given iterator, not counting characters that are invisible due to tags with the "invisible" flag toggled on.