Lets assume your data is x and y Whenever the zoom changes (I don't know how you are implementing your zoom but you may need to add a listener or something else)
xl = xlim %note you may need to use a get command here instead, I am not %100 sure how this will interact with the GUI
%get(handles."your graph",'xlim')
yl=ylim
yt(x<xlim(1) | x>xlim(2) | y<ylim(1) | y>ylim(2))=[];
set(handles."Your Text box",'string',num2str(mean(yt))]
