Error in finding histogram of an image having required bins range [0 255].

I am having an brain image. Following code is used to find histogram (Input image and histogram is attached):

I_1=niftiread('Brats18_2013_5_1_flair.nii.gz');

I=double(I_1);

[R C N]=size(I);

for i=100

img_1=I(:, :, i);

img=fliplr(imrotate(img_1,-90));

H=imhist(img)

end

But I am not getting desired histogram. Kindly provide the solution.