In CREATOR AND CREATED ENSEMBLED HE THEM I sussed out the centricities of various "mad scientists" and their creations. In Stevenson's DOCTOR JEKYLL AND MISTER HYDE, Jekyll's alter ego Hyde has the greatest centricity, and is therefore the story's focal presence. In Wells' ISLAND OF DOCTOR MOREAU, the beast-men creations of the scientist are less central to the story than Moreau himself, and so he takes the position of the focal presence. However, in Shelley's FRANKENSTEIN, both the creator and his creation share the spotlight.
From these books, it should be clear that the title of a feature doesn't indicate the focal presence, and as I've noticed elsewhere, this is equally true in other media. As others before me have noted, the Universal Frankenstein series is principally about the monster, while the Hammer series concentrates on the scientist.
This principle applies across the board to many comics-features. BATMAN started as a concept with just one focal presence. But the addition of Robin, BATMAN became known as an ensemble of two focal presences for the next twenty-odd years. After Robin went away to college, the serial feature frequently alternated between Batman on his own, and Batman rejoined with a new Robin, though some of the Robin-rebirths didn't go so well.
I would tend to say that whenever a comics-feature presented a team-mate as an "equal partner," then that partner, however nugatory he might be as a character, became an equal focal presence in the feature. Yet this sense of equality had to flow more from the creators' attitude toward the character than from the character's representation in the stories. As a contrary example, the comic strip introduced "Junior" to the DICK TRACY in 1932, and the youth got more than a fair number of storylines devoted to him. But he was not treated as an equal partner, and so he remained one of the main character's support-cast.
In the terminology I've introduced here, then, Robin has a transitive effect in terms of his centricity, so that he's centric to the action even in stories where he has no significant role. Junior, though, has an intransitive effect in terms of centricity. Whole story-arcs can be centered on him, but he's never really the focus, but rather a reason for central character Tracy to take action. Tracy is always the "common thread" of the stories, even if he doesn't appear that much in a given arc, much the same way that Will Eisner's Spirit is that feature's common thread even in stand-alone stories where the masked detective barely appears.
Titles of movies and movie-serials are similarly deceptive. CAPTAIN AMERICA: CIVIL WAR picks up story-lines that are established in other movies, particularly AVENGERS: AGE OF ULTRON, and Captain America shares the stage with about eleven other costumed characters. Yet the other Avengers and hangers-on are in the same position as Junior in the DICK TRACY strip: intransitive. The main thrust of the story focuses on two aspects of Captain America's personal cosmos: the fate of his old friend Bucky Barnes, and the need to keep himself and his fellow superheroes free of government oversight (which attitude is to a slight extent justified by the events of INFINITY WAR). The other heroes of CIVIL WAR are more in the nature of "guest stars" than supporting characters-- even the Falcon, who had the status of an equal partner during a brief period of the CAPTAIN AMERICA comic book, but did not achieve that status in the movie series.
But though the title of CAPTAIN AMERICA: CIVIL WAR correctly foregrounds the fact that it's a Captain America film in a series of Captain America movies, AVENGERS: INFINITY WAR is not focused only upon the Avengers in the diegesis. The title in this case only functions to provide a semblance of continuity with the 2012 AVENGERS film, but in structure the story is just as much a sequel to the first GUARDIANS OF THE GALAXY film. INFINITY WAR's structure is directly patterned upon one of Jim Starlin's many superhero smorgasbords, which in turn owes its lineage to early multi-character mashups like Marvel's SECRET WARS. To be sure, not every character in such mashups is necessarily a focal presence. For instance, Shadowcat's quasi-pet Lockheed the Dragon, who was never a focal presence in the X-MEN titles, did not become one just because he also took part in SECRET WARS. He would still be intransitive in terms of centricity, just like Junior Tracy-- but almost every other hero in the story would be a focal presence, whether that hero played a large or small part in the story. (CRISIS ON INFINITE EARTHS also tosses in many hero-cameos that simply don't register high in terms of centricity.)
But INFINITY WAR doesn't have those niggling problems, and so all the featured heroes of the Avengers and the Guardians groups are focal, as is the one solo act, Doctor Strange, making a total of nineteen focal presences in all. The only characters who aren't part of the ensemble are those who weren't ever focal in other films: "helper-types" like Nick Fury, Wong, et al.