While going through some more example codes, I realized 'impulses' style can be used as bar graph with appropriate width option setting. Following code is working as I wanted

   $my_dataset = Chart::Gnuplot::DataSet->new(
         xdata           => \@dates,
         ydata           => \@data,
         style           => 'impulses',
         width           => 15,
         linetype        => 'solid',
         color           => 'green',
         timefmt         => '%Y-%m-%d',
   );
[download]