I doubt that is what you really want.
figure
ax = gca;
x = 0:pi/20:3*pi;
y = cos(x);
plot(ax, x, y);
xticks(ax, x);
xticklabels( ax, string(0:1/20:3) + "\pi");
I think it more likely that you only want occasional axes labels.
figure
ax = gca;
x = 0:pi/20:3*pi;
y = cos(x);
plot(ax, x, y);
xt = x(1:10:end);
xticks(ax, xt);
xticklabels(ax, string(0:1/2:3) + "\pi")
