Assuming the situation where the all data would NOT fit in RAM, I'd suggested using histcouns function to get bin counts of all the text files one by one. Then use bar function to the sum of bin counts to create a histogram.
x = randn(10000,1);
[counts,edges] = histcounts(x); % do this for every file and add-up counts values
center = (edges(1:end-1)+edges(2:end))/2;
bar(center,counts,1)
