How to do a gradient voronoi in 3D

Im new to Grasshopper, and Im trying to do a Voronoi varying the amount of points in a box. By now, I divided a square in four strips and populated 2d with less to more points in each strip.

But I would like to extend this to 3D. I was thinking that I could populate 3D the box, and then divide it by sectors and populate it gradually, or use a Force Field. But Im too new and got stuck in the 2D part. 

Any help or advice is welcome!