[cctbxbb] Electron density hitogram from MTZ file

Hi everybody, I am new in crystallogrphy and in the use of cctbx. I am interested in building a electron density histogram from a mtz file. Could somebody give a hint on how to accomplish this task using cctbx libraries (I am scripting in python)? Let's say that I already have the columns of interest as "miller arrays". Then (I think), a grid (on the real space) needs to be created based on unit cell parameters, to finally make the fft. I think that most of my doubts are about the grid. How small the grid should be with respect to the low-resolution limit? Does the space group information needed or it is enough with the the cell parameters? Is there all ready a routine doing this? Thanks a lot for your help. Marcelino