change histogram to PDF

I have attached a histogram which is attached. This histogram is computed step by step based on milions of samples.

If I don't compute it step-wise, Matlab gives "out of memory" error!

N_bins=100;
epsai= very long cell in which each array has millions of samples
cells_max= max of all samples(#millions)
cells_min= min of all samples(#millions)
bin_width=double(cells_max-cells_min)/N_bins;
edge_epsai=double(cells_min):bin_width:double(cells_max);
N_freq=zeros(1,N_bins);
for i=1:length(epsai)-1
    [N_freq_tmp,~]=histcounts(double(epsai{1,i}),edge_epsai);
    %save frequency
    N_freq=N_freq_tmp+N_freq;
end

result till here is attached, then I want to change to PDF as follows but the result is weird!I get lots of zero and few numbers in beginning anf ending pints of intterval!

[S_y_axis, h3,~]=histcounts(N_freq,edge_epsai,'Normalization','pdf');

Answer 1

[KSSV]

x = randn(10000,1) ; 
histogram(x)

pd = fitdist(x,'kernel');
x_values = min(x):0.01:max(x) ; 
y_values = pdf(pd,x_values);
plot(x_values,y_values)