This is a graph of what members spend on their own military, not what they contribute to NATO. It's still relevant, because we have an obligation to defend other members and how well they defend themselves affects our behavior, but it's not a zero-sum game. Greece decreasing its defense budget does not mean we need to increase ours, and Poland increasing theirs doesn't mean we can decrease ours.

NATO members absolutely should spend what they agreed to spend, but we aren't forced to spend 3.6% of our budget on defense because they don't. That's something we choose to do, and we could stop if we wanted to.