How to normalize surface such that area under surface=1?

I'm trying to plot surface such that area under the surface is equal to 1. I'm using 50x50 histogram (with values from 0 to around 16) to plot a surface which represents probability density distribution. I'm having a hard time to crack this problem and I have absolutely no idea how to normalize my data. Fragment of code below:

dr = 50;                                        % surface 50x50
r = 5;

% yafter and zafter are coordinates of a molecule, there are around 1000
% positions of those and I only wrote below some examples of those
% positions for better understanding of the code, since writing down 
% the whole program would be pointless

yafter = [0.5603 0.5603];       
zafter = [0.8458 0.8458];
N = 2;                                          % number of molecules

for ii=1:N
    position_y = round((yafter(ii)+(2.*r)./dr./2)./(2.*r).*dr)+dr./2+1;
    position_z = round((zafter(ii)+(2.*r)./dr./2)./(2.*r).*dr)+dr./2+1;
    histogram(position_y, position_z) = (histogram(position_y, position_z) + 1);
end
    
figure(3)
hold on
view(3);
surf(histogram);
colormap autumn;
title('Probability density distribution after 10000 steps', 'FontSize', 14);
xlabel('Y', 'FontSize', 24, 'fontweight', 'bold');
ylabel('Z', 'FontSize', 24, 'fontweight', 'bold');
zlabel('P(Y,Z)', 'FontSize', 24, 'fontweight', 'bold');
set(gca, 'FontSize', 16);
axis([0 dr 0 dr]);
hold off