The phrase "long chalk" means by far or by a significant amount. It is used to emphasize that something is much better or greater than another thing. Full definition
Key British works include: Alfred Gilbert Queen Victoria, Phillip King Genghis Khan, Jacob Epstein Adam, Barbara Hepworth Single Form, Leon Underwood Totem to the Artist, Henry Moore Festival Figure, Anthony Caro Early One Morning, Richard Long Chalk Line, Julian Opie W and Damien Hirst Let's Eat Outdoors Today. (museumpublicity.com)
This does not mean that the Gunners are going to storm to the EPL trophy this season, not by a long chalk. (justarsenal.com)
Manchester United have already completed the transfer of the young Dutch forward Memphis Depay and Louis van Gaal is not done by a long chalk. (justarsenal.com)