What you are asking for isn't easy to implement but can be done, what the code has to do is the search for one empty space big enough for that text, there's also a chance that there's no such space available and the code will fail.
There are alternatives like using the legend, xlabel, ylabel and title. Also title can have many lines
title({'Main Title text'; 'Subtitle text'});
or this solution:
title(['\fontsize{14} Main Title ', ...
'\newline \fontsize{10} \color{red} \it My Subtitle']);
There is also the gtext function that allows the user to select where to put the text.
