Problem with graph continuity

I attached file with solution for beam on elastic foundation using Bleich method. I am struggling with the graph of shear forces. When i use dimensionless variable xi for graph plotting everything is just fine but once i convert the dimensionless variable to one in meters ( dividing dimensionless variable by certain coefficient alfa, relation defined at the beginnig of the worksheet) i am getting discontinuity of the graph and have no idea why. Also trying to find place using find function where Q(xi) equals zero( should be between two peaks) i result in failure. If anyone could provide any helpful advice/answer i would really appreciate it.