When you inhale, air goes into the lungs. That air comes from the bucket. As the air volume in the bucket decreases, the bucket sinks
As the pressure in the bucket is decreased, the atmospheric pressure will push fluid into the bucket to replace this air volume so that the pressure inside the bucket remains equal to the pressure outside. So, the bucket sinks and pulls at the string!
) and pulls on the string. This string pulls, through a series of wheels, the pencil that draws a curve on the graph.