Say you are defining a filter in GNU Octave. You need the frequency on the horizontal axis to be logarithmic instead of linear. This is done using the semilogx() function. In the next step you can see a high pass filter (HP) that demonstrates just that and generates the above image.
Step 1: Semilogx()
freq = [20:100:20000];
xc = 1 ./ (2 * pi * 0.000000016 * freq); z = sqrt(xc.^2 + 1000^2); vout = 10.0 * 1000 ./ z; semilogx(freq, vout, ‘r-’); grid on; xlabel(‘freq’); ylabel(‘volts’); legend(‘HP filter’);