Pixel value difference histogram of two images

Suppose I have two matrices for cover and stego images. I want to draw the pixel value difference histogram as shown in the above figure. I am facing problem in setting edges. I need the x-label as [-20 -15 -10 -5 0 5 10 15 20] as shown in the above figure. I could not able to show the Legents as the above figure. Please help me.....

edges = [-20 -5:0:5 20];

histogram(cover,edges,'DisplayStyle','stairs');

hold on;

histogram(stego,edges,'DisplayStyle','stairs');

legend('Cover Image');

legend('Stego Image');