define_method :"#{method}_by_author_by_date" do |authors = nil|
            Chart.new do |f|
              authors ||= @authors.sort_by { |a| -a.send(method) }[0...AUTHORS_ON_CHART_LIMIT]
              f.multi_date_chart(
                data: authors.map { |a| {name: a.name, data: a.send(:"#{method}_by_date")} },