Yes, that graph shows the longer UK waited to fight, the worse off they became, and the war was inevitable anyway, unless they wanted to become a German vassal. That was Churchill's point.
He was going to use poison gas if Germans invaded Britain also. Everyone used poison gas in WWI, and the only reason they didn't in WW2 was fear of retaliation, not moral principles. Churchill made many mistakes, but he also did many things right and brilliantly, he was a real risk taker. For example he was instrumental in the development of the tank, and came up with the floating harbors, the Mulberrys, used in the Normandy invasion. As far as Gallipoli, he was more responsible for the Dardanelles, which was the naval operation, while Gallipoli was a land operation, since he was the First Lord of the Admiralty. Even there, if the war cabinet made the decision to attack Dardanelles when Churchull first proposed it, and thus the Turks wouldn't had the time to mine them, the operation would've succeeded. But no question Churchill had a lot of hare brained ideas. Someone said he had 100 ideas a day, 4 of which were good. And of course his leadership did save Britain, when everyone thought they were finished, and he was a very strong leader, certainly not a figurehead at all.