It's more than the matching scales. The way the scales adjust on-the-fly means that once each graph (inevitably) either hits the near-top or near-bottom of the

It's more than the matching scales. The way the scales adjust on-the-fly means that once each graph (inevitably) either hits the near-top or near-bottom of the graph, the line stays there, and doesn't appear to move as the scale adjusts around it - the scale adjusts to keep the line close to the "adjustment point" near the edge of the graph.