How to centre textboxes on the plot outline?

Hi, I'm trying to create a plot with a couple of textbox annotations. Ideally, I'd like the text boxes to be centred on the boundary of the white plot area - not relative to the figure space. However, I'm struggling to find a way to easily do this - is there a means of finding these dimensions and not the dimensions of the entire figure? My code is as follows:

%Illustrates wavelength vs period
close all;

   Fs = 8000;                   % samples per second
   dt = 1/Fs;                   % seconds per sample
   StopTime = 0.25;             % seconds
   t = (0:dt:StopTime-dt)';     % seconds
   %%Sine wave:
   Fc = 4;                     % hertz
   x =sin(2*pi*Fc*t);
   % Plot the signal versus time:
   figure;
   plot(t,x);
   set(gca,'xtick',[])
   set(gca,'ytick',[])
   
   annotation('doublearrow',[0.132142857142857,0.903571428571429],[0.923,0.923])
   str = {'Period','T'};
   dim = [0.457785714285714,0.883333333333335,0.127571428571429,0.0952380952381];
   annotation('textbox',dim,'String',str,'FitBoxToText','on','Horizontalalignment','center','BackgroundColor', 'w');
   
   annotation('doublearrow',[0.132142857142857,0.9035714285714291],[0.11,0.11])
   str = {'Wavelength','\lambda'};
   dim = [0.457785714285714,0.054761904761905,0.127571428571429,0.0952380952381];
   annotation('textbox',dim,'String',str,'FitBoxToText','on','Horizontalalignment','center','BackgroundColor', 'w');
   
   hold on;
   yline(0);
   zoom xon;

I hope what I am trying to achieve is clear from the plot, but as you can see it uses a very convoluted way of achieving a result which still doesn't look right.

Thanks in advance for your time.

Answer 1

[Jakob B. Nielsen]

First, make a handle for your figure:

   ax=axes;
   plot(ax,t,x);

Next, when you set the dimensions of your textbox, refer back to the ax.Position property:

   dim = [(ax.Position(1)+ax.Position(3))/2,(ax.Position(2)+ax.Position(4)-0.0952380952381),0.127571428571429,0.0952380952381];

Here, I make sure the x start point of the box is the x-startpoint of the axes plus width of the axes divided by two (midway), and that the y start point of the box is the y start point of the axes plus the height of the axes, minus the height of the text box. See if you can make the same for the other box :)

[Ted Baker]

Thank you Jakob!