how to use datetick?

hi,....i hve a file...first column is date and time second is data.file attached i using following code..it showing me error...i dont know why.and more over i want date tick at regular 3 month interval...not randomly...

x= vcdmean(:,1);
y = vcdmean(:,2);
plot(x,y,'.')
datetick('x', 'mmm YYYY')

Answer 1

[Azzi Abdelmalek]

q=datevec(x)
[~,ii]=unique(q(:,1:2),'rows')
a=q(ii,:)
a=a(1:3:end,:)
xt=datenum(a)
plot(x,y,'.')
set(gca,'xtick',xt)
datetick('x', 'mmm YYYY','keepticks')

[pruth]

accepted .....thanks...