I would go as far as to say Xavier was the main character of DoFP clearly.
You'd be correct.
The X-Films tend to have a main plot for the team and sub-plot delving into an individual character.
X-MEN
Main Plot:
Introducing and outlining Mutants place in the world with Magneto plotting to turn the world leaders into Mutants.
Sub Plot:
Rogue manifesting her powers and her reaction to them with her trying to find a safe haven and accept what she is.
X2: X-Men United
Main Plot:
Col. William Stryker using Xavier to try to eradicate all Mutants with the X-Men uniting with their enemies to stop this greater threat.
Sub Plot:
Wolverine trying to learn of his past and deciding to cease that search to embrace who is is now rather than finding out who's he was then.
X-Men 3: The Last Stand
Main Plot:
The Government know that Mutants are a threat to Mankind but they can't just kill them as it would be Inhumane, so they develop and cure which at first they offer but then force upon Mutants.
Sub Plot:
Jean Grey returns as Phoenix a Mutant with too much power and not enough humanity, while Magneto uses her the X-Men must kill their former teammate for the greater good.
X-Men First Class
Main Plot:
Charles Xavier and Erik Lensherr work with the CIA and some young Mutant recruits to stop Sebastian Shaw from starting World War 3.
Sub Plot:
Erik Lensherr hunts down the main who murdered his mother and experimented on him as a child, along the way he becomes Magneto, protector of Mutantkind and scourge of Humanity
X-Men: Days of Future Past
Main Plot:
The future X-Men send Wolverine back in time in a last ditch attempt to save the future for Human and mutants alike
Sub Plot:
Charles Xavier a broken and addicted Man comes to accept and embrace his destiny of becoming leader of Mutantkind