[Kwant] How to feed a portion of a finite system to kwant.plotter.interpolate_density

Dear Kwant developers, I have a finalized closed system ''sys_f'' for which I computed the local charge densities "ch_densities", which is basically a vector of length=len(sys_f.sites). For complexity reason,  I'd like to feed a slice of sys_f  to the interpolate_density() as:

...

kwant.plotter.interpolate_density(syst=sys_p, density=ch_densities_p)

where sys_p is a slice of sys_f and ch_densities_p is the corresponding density vector.Could you please let me know how to do it? Best regards,Hadi