how to plot graph

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);
hist(P+Q+R+S+T,i);

I want to plot a graph beween partial derivative of s(whereas s varies from 0.0 to1.0) and i

Thanks.