sgrid — s-plane grid lines.
sgrid()
sgrid('new')
sgrid(zeta,wn [,color]) Used in conjonction with evans,
plots lines of constant damping ratio (zeta)
and natural frequency (wn).
add a grid over an existing continuous s-plane root with default values for zeta and wn.
clears the graphic screen and then plots a default s-plane grid
same as sgrid() but uses the provided damping ratio and natural frequency.