Root-locus (and sisotool root-locus) used to recompute their points when zooming in on the root-locus plot (typically to get to the dominant pole group). At zoom-in, the new xlim and ylim are used to recompute the RL lines.

Currently root-locus does not recompute, and zoomed-in plots show (sometimes unusably) coarse edgy root-locuses. The recompute was removed in commit

57dac87da3c5885ef4912fa4484a0dff811e0b9c    Initial refactoring of root_locus_{map,plot}

I am currently trying to figure out how to bring recalculation of the plot back.

Example:

import control.matlab as ml
from matplotlib import pyplot as plt

s = ml.tf('s')
H = (s+1)/s**2/(s**2+2*s+7)/(s+15)
ml.rlocus(H)
plt.show()