Data exercise
=============

Look at `this page from the Particle Data Group <http://pdg.web.cern.ch/pdg/2013/hadronic-xsections/hadron.html>`_. It has plots and data files for the *total collision cross section* of :math:`\pi^-p` and :math:`\pi^+p` collisions. Instead of the two separate plots, we would like to have both lines on a single figure.

Using the command line tools we discussed in the previous sessions, you should
download the data files, extract the relevant columns and produce a
plot using :command:`gnuplot`. We'll introduce other
plotting tools for data sets later.

Gnuplot expects a data file with one plot point per line, in six columns: each line should contain the points with error bar information in the format
:kbd:`x y xlow xhigh ylow yhigh`. Convert the data files from the PDG
website into this format. (:command:`awk` from previous weeks will be useful, it lets you calculate with column references: :kbd:`$2 + $3`).

After converting the data into the correct format, a simple :command:`gnuplot` session looks like::

  $ gnuplot
  [... some splash text ...]
  gnuplot> set logscale
  gnuplot> plot 'filename1' with xyerrorbars, 'filename2' with xyerrorbars
  gnuplot> quit

Instead of entering these instructions into a gnuplot session one by one, you can also save them in a file, say :file:`plot.gnu`, and call::

  $ gnuplot plot.gnu

To create the plot in a PDF file which you can import in a
:math:`\LaTeX` document, call::

  set terminal pdf
  set output 'foo.pdf'

before the plot command. :command:`gnuplot` has very detailed information
accessible through its :kbd:`help` command.