You are right. There is no way of which I'm aware how to rotate the diagram plotting plane - it is a good candidate for enhancement. The plotting has to do with the orientation of the BEAM axes. You can see the BEAM axes by right-clicking on the icon representing the shear/bending diagram and choose "Settings...", then choose "show beam directions". This will show how the orientation of the shear/bending diagram will appear. Here is a link to a short video on GoEngineer's YouTube channel for Beam Shear and Moment Diagrams. I hope it will help, at least to explain how they are plotting it.