I understand that you would like to have multiple titles to a plot corresponding to the firing rate of various segments of that plot. You may try using the text and annotation commands to place text on the plot at appropriate position by providing the co-ordinates of the position at which the text is to be placed to those functions. Here's an example using text command:
figure(1)
plot(t, Si)
text(1, 8, '120 Hz') % Here (1,8) represents the co-ordinates at which '140 Hz' text would be placed.
text(3, 8, '140 Hz')
text(5, 8, '150 Hz')
text(7, 8, '100 Hz')
You can also implement this using the 'Edit plot' feature of Figure window ( Tools -> Edit Plot ) to move the postion of text boxes as per your convenience to make the textboxes look as titles for the respective segements. An illustration of this idea is shown in the figure below:
