def plot_histo_clusters(clusters = @clusters,
                            max_density_cluster = @max_density_cluster,
                            prediction = @prediction)

      data = clusters.each_with_index.map do |cluster, i|
        cluster.lengths.collect do |k, v|
          { 'key' => k, 'value' => v, 'main' => (i == max_density_cluster) }
        end
      end

      Plot.new(data,
               :bars,
               'Length Cluster Validation: Distribution of BLAST hit lengths',
               'Query Sequence, black;Most Dense Cluster,red;Other Hits, blue',
               'Sequence Length',
               'Number of Sequences',
               prediction.length_protein)
    end
def plot_histo_clusters(clusters = @clusters,
                            max_density_cluster = @max_density_cluster,
                            prediction = @prediction)

      data = clusters.each_with_index.map do |cluster, i|
        cluster.lengths.collect do |k, v|
          { 'key' => k, 'value' => v, 'main' => (i == max_density_cluster) }
        end
      end

      Plot.new(data,
               :bars,
               'Length Cluster Validation: Distribution of BLAST hit lengths',
               'Query Sequence, black;Most Dense Cluster,red;Other Hits, blue',
               'Sequence Length',
               'Number of Sequences',
               prediction.length_protein)
    end
Generates a json file containing data used for plotting the histogram
 of the length distribution given a lust of Cluster objects
 +output+: plot_path where to save the graph
 +clusters+: array of +Cluster+ objects
 +max_density_cluster+: index of the most dense cluster
 +prediction+: +Sequence+ object
 Output:
 +Plot+ object