When you zoom in on a region, you might not include any part of the x-axis or the y-axis. CalGraph wants you to have some idea of the values of x and y so it draws an axis for you, and labels it. This new axis is drawn with dotted lines; that way you know that where the axes cross is not the origin.
You can zoom in further on the zoomed graph, by clicking Zooming and then Zoom In.
You can go back to the previous graph by clicking Zooming and then Reverse Last Zoom.
You can zoom out too.