def label_width
      if !@label_width then
        @label_width = @samples.find_all{ |s| Sample === s }.
                          max{ |a, b| a.label.length <=> b.label.length }.
                          label.length + 1

        @label_width = 40 if @label_width < 40
      end

      return @label_width
    end
def label_width
      if !@label_width then
        @label_width = @samples.find_all{ |s| Sample === s }.
                          max{ |a, b| a.label.length <=> b.label.length }.
                          label.length + 1

        @label_width = 40 if @label_width < 40
      end

      return @label_width
    end
Calculate the label padding, taking all labels into account