Call <a href="http://www.gnuplot.info/">gnuplot</a>
 to plot the histogram of atom energy
i.e. <tt>EPOT_IND</tt> (by default).

<p>
If <tt>plot_color_axis = 2</tt> then the histogram of
<tt>TOPOL</tt> is plotted instead.  

The array <tt>TOPOL</tt> stores the computes the central symmetry 
deviation (CSD) parameters for each atom
computed by <tt>calcentralsymmetry</tt>.
