Adding histograms to plotting chapter

[leios]

Feature Request

Description

With #929, we noticed we needed a histogram plotting method. To do this in gnuplot, it is something like this:

bin_width = 1.0; ## edit this 
bin_number(x) = floor(x/bin_width)
rounded(x) = bin_width * ( bin_number(x) + 0.5 )
UNITY = 1
## column number of data to be histogrammed is here assumed to be 1
## - change $1 to another column if desired
plot 'output.dat' u (rounded($1)):(UNITY) t 'data' smooth frequency w histeps

from: https://wikis.ch.cam.ac.uk/ro-walesdocs/wiki/index.php/Plotting_a_quick_histogram_in_gnuplot_using_the_raw_data