Combining histograms of images

I am wanting to combine the histograms of 10 images (12 bit grayscale). Due to image processing prior, although the max level should be 4095, there is some PRNU correction so each image has a slightly different max value and one that is bigger than 4095 by a small number.

I currently read each image in a foor loop

for i=1:10
IM=imread(file)
[maxval,idx]=max(IM(:));
maxval= double(maxval);
[counts,x] = imhist(HX,maxval);
%Assign each counts and x to an index array
HX(i)=counts
HY(i)=x
end

%Then to view the composite histogram:

 figure
 stem(HY,HX,'b', '.');

But this don't work.

[Jason]

just to add, to account for the small variation in the max grayscale level, i have redefined maxval as the maxlevel plus 10%. I've also tried just adding counts from histogram, but Im making a mistake somewhere.

[maxval,idx]=max(IM(:));
 maxval= double(maxval*1.1)
 [counts,x] = imhist(IM,maxval)
 Totalcounts=Totalcounts+counts;