This seems like a more straightforward and correct way of implementing sgrid - can you confirm sisotool still works as you might expect on that transfer function, and that the gridlines turn off when rlocus_grid=False?

Also, we currently have an excess of sgrid functions. There is another : ) It is to be found in grid.py and it is called sgrid. I haven't compared the two but that one may be more modern/faster? (though it also needs to be upgraded to accept an ax keyword). If so, maybe the root_locus function should switch to using that one and delete the one in rlocus.