Gnuplot - if you want help ...

Gnuplot plots in LaTeX
http://psung.blogspot.com.es/2007/03/gnuplot-plots-in-latex.html

User guide
http://www.gnuplot.info/docs_4.6/gnuplot.pdf

How to do histograms - fantastic
http://gnuplot.sourceforge.net/demo/histograms.html
http://www.codealias.info/technotes/gnuplot_cluster_histogram_example

Many other examples
http://gnuplot.sourceforge.net/demo/datastrings.html
http://gnuplot.sourceforge.net/demo/stringvar.html
http://psy.swansea.ac.uk/staff/Carter/gnuplot/gnuplot_histograms.htm
http://gnuplot-surprising.blogspot.com.es/2011/09/plot-histograms-using-boxes.html

More complex
http://www.phyast.pitt.edu/~zov1/gnuplot/html/histogram.html