Is this a physical line (in a factory, say)? or a line of print (a list)? Normally, I would say "on." An object cannot be in (within) a line. It can be on (on top of).
In a list, where you are talking about a "line" as a row of words, you might use either "on" or "in." However, even there, I believe, the common choice would be "on."