??? Index exceeds matrix dimensions.

Actually I am trying to plot the local maxima on a data which is a matrix but i get this error '??? Index exceeds matrix dimensions.' here is my code

   surf(T,f,G'), shading interp
      [Gmax,imax,Gmin,imin] = extrema2(G);
      hold on  
       plot3(T(imax),f(imax),Gmax,'bo')
       for i = 1:length(Gmax)
        text(x(imax(i)),y(imax(i)),zmax(i),['  ' num2str(zmax(i))])
       end
       hold off

the error is in the line ' plot3(T(imax),f(imax),Gmax,'bo')' how can i change the size of the vector T ad imax is very large this is what creates the problem. The extreme2 function can be found in this link 'http://www.mathworks.com/matlabcentral/fileexchange/12275'