(i) as to how to draw the VD- ID, VG-ID etc. by kwant coding. I am to some extent clear to get those relations from the back calculations of G= ID/VG and the current expression in terms of the G. Finding G by kwant they may be found out. But, I like very much a direct coding technique to get these curves.
I believe that getting a differential conductance using Kwant and then integrating it numerically is the only approach in this case.
(ii) as to how to visualize a 3D graphics of nanostructure like QWR, QD which I visualize in 2D without problem.
You can either use kwant.plot using a 3D system, or any other 3D plotting library if the capacity of kwant.plot is insufficient.
Best regards, Anton Akhmerov