How to make equal rows to plot histogram

 RGB =imread('image.jpg');
hsv= rgb2hsv(RGB);
imshow(hsv);
r1=0.1;
r2=0.85;
i= 0:20:240;
s= 0.0:0.2:1.0;
wgray = 1 - bsxfun(@power, s(:), (r1*(255./(i(:).')).^r2) ); 
P = gradient(wgray,0.2);
Q= gradient(wgray,0.4);
R= gradient(wgray,0.6);
S= gradient(wgray,0.8);
T= gradient(wgray,1.0);
imhist(P,i);

The matrix P and I are of unequal length so getting error in plotting histogram....I am new to matlab so need help

Thanks