问个matlab semilogy的问题

constant007

clc;clear all;close all;
f=-4:0.001:4;
X=abs(sinc(f));
semilogy(f,X);
%axis([-4 4 0.001 1.1]);
%set(gca,'YTickLabel',{'-60' '-40' '-20' '0'})
xlabel('Frequency(Hz)');
ylabel('Amplitude(dB)');
grid on

为什么画出来y轴都是正数呢;

X是[0 1]  
log10(X)  应该是负数;
为啥用semilogy画出来是正数呢