The graph is the transmitted power in the incident mode, LP(0,1). In the Fiber Modes dialog box, there are 5 other modes selected: LP(0,2) to LP(0,6). As the wavelength changes, the grating period of 450 microns will sometimes be the right period to couple light from LP(0,1) to one of the other modes. If coupling occurs, the power is lost from the LP(0,1) mode and it appears in the graph as a dip.