Control the display of z-plane grid with :

• zeta lines corresponding to damping ratios and
• omega lines corresponding to undamped natural frequencies

The function state input may be either "on" or "off" for creating or removing the grid. If omitted, a new grid is created when it does not exist or the visibility of the current grid is toggled.

The zgrid will automatically plot the grid lines at nice values or at constant values specified by two arguments :

where Z and W are :

• Z vector of constant zeta values to plot as lines
• W vector of constant omega values to plot as circles

Example of usage:

 zgrid on	  create the z-plane grid
 zgrid off 	remove the z-plane grid
 zgrid		  toggle the z-plane grid visibility
 zgrid ([0.3, 0.8, ...], [0.25*pid, 0.5*pi, ...])   create:
 

 zgrid (hax, "on")   create the z-plane grid for the axis
                     handle hax