Try something like this —
x = linspace(0, 21, 25);
y = rand(size(x));
figure
plot(x, y)
xt = get(gca, 'XTick');
xtv = linspace(min(xt), max(xt), 21);
set(gca, 'XTick',xtv, 'XTickLabel',compose('%3d dB',-20:2:20))
Make appropriate changes to get the result you want.
.