a = 0;
b = 20;
x = a:0.1:b;
y = sin(x);
plot(x,y)
osax = a:0.5:b;
set(gca,'XTick',osax)
ymin = min(y);
ymax = max(y);
osay = linspace(ymin,ymax,b);
set(gca,'YTick',osay)
set(gca,'XTick',0:pi/2:b)
set(gca,'XTickLabel',{'0','pi','2pi', '3pi'})

n = floor((b-a)/(pi/2))+1;
popisek = cell(1,n);
popisek{1} = num2str(a);
for i=2:n
    popisek{i} = [num2str(i) '/2*pi'];
end

set(gca,'XTickLabel',popisek)
