add a x axis tick list with selected frequencies

sphear_fir_render.m

@@ -143,6 +143,7 @@ function [BFF] = sphear_fir_render(FIR, fmin, fmax, N, Npre, Fs, plt)
       title(sprintf("azimuth = %05.1f degrees", angle));
       axis([300, 20000, -60, 6]);
       tics('y', [6, 0, -2.5, -5, -7.5, -10, -12.5, -15, -17.5, -20, -22.5, -25, -30, -40, -50]);
+      tics('x', [300, 500, 1000, 2000, 3000, 5000, 7000, 10000, 15000, 20000]);
       semilogx(fr, wcr(1:end/2), 'k;W;');
       semilogx(fr, xcr(1:end/2), 'r;X;');
       semilogx(fr, ycr(1:end/2), 'b;Y;');