I updated the web: Problems and examples of frequency response (Bode
diagram, Nyquist diagram, phase margin, gain margin, lead compensator, lag
compensator, lag-lead, Nichols chart, static velocity constant error, 
resonant peak magnitude, resonant peak frequency, bandwidth) by Scilab
(bode, syslin, logspace, nyquist, g_margin, p_margin, show_margins,repfreq,
dbphi, tf2ss, chart, black, m_circle, horner)

http://cpbas.webcindario.com/eng/control/FrequencyResponse/index.php