The following code

 w=0:0.01:1000;
 [z, p, k] = cheb2ap (3, 3);
 [b, a] = zp2tf (z, p, k);
 Gs = freqs (b, a, w);
 semilogx (w, abs (Gs));
 xlabel('Frequency in rad/sec')
 ylabel('Magnitude of G(s)');
 title('Type 2 Chebyshev Low-Pass Filter, k=3, 3 dB ripple in stop band')
 grid;

Produces the following output

warning: axis: omitting non-positive data in log plot
warning: called from
    __plt__>__plt2vv__ at line 502 column 10
    __plt__>__plt2__ at line 248 column 14
    __plt__ at line 115 column 16
    semilogx at line 65 column 10
    get_output at line 50 column 5
    __html_help_text__ at line 67 column 28
    generate_package_html>wrote_html at line 844 column 5
    generate_package_html at line 209 column 11

and the following figure

Figure 1