SAN FRANCISCO — The flat screen TVs in the Giants clubhouse at Oracle Park often show clips of that night’s opposing pitcher, giving hitters a chance to do a little last-minute homework as they get dressed at their locker or walk through the room. But when the Giants returned home Monday after a winless trip, some of the TVs showed a small graphic.There was a box with OBP on one side and the number .287 underneath. On the other side of the box, there was the team’s ranking in on-base percentage: 30.The message was not exactly subtle.The Giants returned home ranked dead-last in the big leagues in on-base percentage, and they dipped even lower over three games against the San Diego Padres. It’s not hard to find the reason for their struggles getting on base, either.They are 18th in MLB with a teamwide batting average of .238, but they’re last in the majors in walks and it’s not even close. They’re on pace to be historically bad at collecting free passes, an issue that is threatening to derail an entire season.There are a lot of things — or players — you could point to, but for president of baseball operations Buster Posey,...