In the continuous-time root locus, the straight grid lines that radiate from the origin represent lines of constant damping ratios. By default, the numbers next to grid lines are the damping ratios. In the discrete-time case, the grid lines are closed curves originating from (1,0). You can display the damping ratios as a percentage of peak overshoot by using the Options pane of the Property Editor for root locus in the SISO Design Tool.