Rotate Grid About Specified Point

To give you some background, I'm just starting to use Grasshopper and I have not used Rhino3D in many years.  

Ultimately I would like to create a customizable 3D grid that can be twisted.  Each vertical line would become a separate column element at each story level and each horizontal line would become a beam element.  It could then be exported to analysis software of Revit.  

Most of this is well beyond my understanding as of yet.  But I'm getting hung up at a particular point regard the plane rotation.  I have been successful in getting a top and bottom grid, and having the top grid rotate based on a slider.  However, the grid rotates at the origin of the grid which is at the lower left in my image.  I would like to adjust this so that it always rotates at the center of the grid.  I was able to use some math functions to modify the plane origin, but it just shifts the entire plane and not just the origin.  

Is there a way to easily shift the grid based on the inputs I already have?  

Thanks!