diff --git a/lib/youplot.rb b/lib/youplot.rb
index ce379d6..78f0bad 100644
--- a/lib/youplot.rb
+++ b/lib/youplot.rb
@@ -3,7 +3,6 @@
 require 'unicode_plot'
 require 'youplot/version'
 require 'youplot/preprocessing'
-require 'youplot/plot'
 require 'youplot/command'
 
 module YouPlot
diff --git a/lib/youplot/backends/unicode_plot_backend.rb b/lib/youplot/backends/unicode_plot_backend.rb
new file mode 100644
index 0000000..9ead9b2
--- /dev/null
+++ b/lib/youplot/backends/unicode_plot_backend.rb
@@ -0,0 +1,167 @@
+# frozen_string_literal: true
+
+require 'unicode_plot'
+
+module YouPlot
+  # plotting functions.
+  module Backends
+    module UnicodePlotBackend
+      module_function
+
+      def barplot(data, params, count: false)
+        headers = data.headers
+        series = data.series
+        # `uplot count`
+        if count
+          series = Preprocessing.count_values(series[0])
+          params.title = headers[0] if headers
+        end
+        if series.size == 1
+          # If there is only one series, use the line number for label.
+          params.title ||= headers[0] if headers
+          labels = Array.new(series[0].size) { |i| (i + 1).to_s }
+          values = series[0].map(&:to_f)
+        else
+          params.title ||= headers[1] if headers
+          labels = series[0]
+          values = series[1].map(&:to_f)
+        end
+        UnicodePlot.barplot(labels, values, **params.to_hc)
+      end
+
+      def histogram(data, params)
+        headers = data.headers
+        series = data.series
+        params.title ||= data.headers[0] if headers
+        values = series[0].map(&:to_f)
+        UnicodePlot.histogram(values, **params.to_hc)
+      end
+
+      def line(data, params)
+        headers = data.headers
+        series = data.series
+        if series.size == 1
+          # If there is only one series, it is assumed to be sequential data.
+          params.ylabel ||= headers[0] if headers
+          y = series[0].map(&:to_f)
+          UnicodePlot.lineplot(y, **params.to_hc)
+        else
+          # If there are 2 or more series,
+          # assume that the first 2 series are the x and y series respectively.
+          if headers
+            params.xlabel ||= headers[0]
+            params.ylabel ||= headers[1]
+          end
+          x = series[0].map(&:to_f)
+          y = series[1].map(&:to_f)
+          UnicodePlot.lineplot(x, y, **params.to_hc)
+        end
+      end
+
+      def get_method2(method1)
+        "#{method1}!".to_sym
+      end
+
+      def plot_xyy(data, method1, params)
+        headers = data.headers
+        series = data.series
+        method2 = get_method2(method1)
+        series.map! { |s| s.map(&:to_f) }
+        if headers
+          params.name   ||= headers[1]
+          params.xlabel ||= headers[0]
+        end
+        params.ylim ||= series[1..-1].flatten.minmax # why need?
+        plot = UnicodePlot.public_send(method1, series[0], series[1], **params.to_hc)
+        2.upto(series.size - 1) do |i|
+          UnicodePlot.public_send(method2, plot, series[0], series[i], name: headers&.[](i))
+        end
+        plot
+      end
+
+      def plot_xyxy(data, method1, params)
+        headers = data.headers
+        series = data.series
+        method2 = get_method2(method1)
+        series.map! { |s| s.map(&:to_f) }
+        series = series.each_slice(2).to_a
+        params.name ||= headers[0] if headers
+        params.xlim = series.map(&:first).flatten.minmax # why need?
+        params.ylim = series.map(&:last).flatten.minmax  # why need?
+        x1, y1 = series.shift
+        plot = UnicodePlot.public_send(method1, x1, y1, **params.to_hc)
+        series.each_with_index do |(xi, yi), i|
+          UnicodePlot.public_send(method2, plot, xi, yi, name: headers&.[]((i + 1) * 2))
+        end
+        plot
+      end
+
+      def plot_fmt(data, fmt, method1, params)
+        case fmt
+        when 'xyy'
+          plot_xyy(data, method1, params)
+        when 'xyxy'
+          plot_xyxy(data, method1, params)
+        else
+          raise "Unknown format: #{fmt}"
+        end
+      end
+
+      def lines(data, params, fmt = 'xyy')
+        check_series_size(data, fmt)
+        plot_fmt(data, fmt, :lineplot, params)
+      end
+
+      def scatter(data, params, fmt = 'xyy')
+        check_series_size(data, fmt)
+        plot_fmt(data, fmt, :scatterplot, params)
+      end
+
+      def density(data, params, fmt = 'xyy')
+        check_series_size(data, fmt)
+        plot_fmt(data, fmt, :densityplot, params)
+      end
+
+      def boxplot(data, params)
+        headers = data.headers
+        series = data.series
+        headers ||= (1..series.size).map(&:to_s)
+        series.map! { |s| s.map(&:to_f) }
+        UnicodePlot.boxplot(headers, series, **params.to_hc)
+      end
+
+      def colors(color_names = false)
+        UnicodePlot::StyledPrinter::TEXT_COLORS.each do |k, v|
+          print v
+          print k
+          unless color_names
+            print "\t"
+            print '  ●'
+          end
+          print "\033[0m"
+          print "\t"
+        end
+        puts
+      end
+
+      def check_series_size(data, fmt)
+        series = data.series
+        if series.size == 1
+          warn 'youplot: There is only one series of input data. Please check the delimiter.'
+          warn ''
+          warn "  Headers: \e[35m#{data.headers.inspect}\e[0m"
+          warn "  The first item is: \e[35m\"#{series[0][0]}\"\e[0m"
+          warn "  The last item is : \e[35m\"#{series[0][-1]}\"\e[0m"
+          exit 1
+        end
+        if fmt == 'xyxy' && series.size.odd?
+          warn 'YouPlot: In the xyxy format, the number of series must be even.'
+          warn ''
+          warn "  Number of series: \e[35m#{series.size}\e[0m"
+          warn "  Headers: \e[35m#{data.headers.inspect}\e[0m"
+          exit 1
+        end
+      end
+    end
+  end
+end
diff --git a/lib/youplot/command.rb b/lib/youplot/command.rb
index 3a6c78f..8d392ac 100644
--- a/lib/youplot/command.rb
+++ b/lib/youplot/command.rb
@@ -3,6 +3,9 @@
 require_relative 'preprocessing'
 require_relative 'command/parser'
 
+# FIXME
+require_relative 'backends/unicode_plot_backend'
+
 module YouPlot
   Data = Struct.new(:headers, :series)
 
@@ -11,8 +14,9 @@ module YouPlot
     attr_reader :data, :fmt, :parser
 
     def initialize
-      @params = Params.new
-      @parser = Parser.new
+      @params  = Params.new
+      @parser  = Parser.new
+      @backend = YouPlot::Backends::UnicodePlotBackend
     end
 
     def run
@@ -28,7 +32,7 @@ module YouPlot
       @debug    = parser.debug
 
       if command == :colors
-        Plot.colors(parser.color_names)
+        @backend.colors(parser.color_names)
         exit
       end
 
@@ -39,21 +43,21 @@ module YouPlot
         pp @data if @debug
         plot = case command
                when :bar, :barplot
-                 Plot.barplot(data, params)
+                 @backend.barplot(data, params)
                when :count, :c
-                 Plot.barplot(data, params, count: true)
+                 @backend.barplot(data, params, count: true)
                when :hist, :histogram
-                 Plot.histogram(data, params)
+                 @backend.histogram(data, params)
                when :line, :lineplot
-                 Plot.line(data, params)
+                 @backend.line(data, params)
                when :lines, :lineplots
-                 Plot.lines(data, params, fmt)
+                 @backend.lines(data, params, fmt)
                when :scatter, :s
-                 Plot.scatter(data, params, fmt)
+                 @backend.scatter(data, params, fmt)
                when :density, :d
-                 Plot.density(data, params, fmt)
+                 @backend.density(data, params, fmt)
                when :box, :boxplot
-                 Plot.boxplot(data, params)
+                 @backend.boxplot(data, params)
                else
                  raise "unrecognized plot_type: #{command}"
                end
diff --git a/lib/youplot/plot.rb b/lib/youplot/plot.rb
deleted file mode 100644
index 2bb4347..0000000
--- a/lib/youplot/plot.rb
+++ /dev/null
@@ -1,165 +0,0 @@
-# frozen_string_literal: true
-
-require 'unicode_plot'
-
-module YouPlot
-  # plotting functions.
-  module Plot
-    module_function
-
-    def barplot(data, params, count: false)
-      headers = data.headers
-      series = data.series
-      # `uplot count`
-      if count
-        series = Preprocessing.count_values(series[0])
-        params.title = headers[0] if headers
-      end
-      if series.size == 1
-        # If there is only one series, use the line number for label.
-        params.title ||= headers[0] if headers
-        labels = Array.new(series[0].size) { |i| (i + 1).to_s }
-        values = series[0].map(&:to_f)
-      else
-        params.title ||= headers[1] if headers
-        labels = series[0]
-        values = series[1].map(&:to_f)
-      end
-      UnicodePlot.barplot(labels, values, **params.to_hc)
-    end
-
-    def histogram(data, params)
-      headers = data.headers
-      series = data.series
-      params.title ||= data.headers[0] if headers
-      values = series[0].map(&:to_f)
-      UnicodePlot.histogram(values, **params.to_hc)
-    end
-
-    def line(data, params)
-      headers = data.headers
-      series = data.series
-      if series.size == 1
-        # If there is only one series, it is assumed to be sequential data.
-        params.ylabel ||= headers[0] if headers
-        y = series[0].map(&:to_f)
-        UnicodePlot.lineplot(y, **params.to_hc)
-      else
-        # If there are 2 or more series,
-        # assume that the first 2 series are the x and y series respectively.
-        if headers
-          params.xlabel ||= headers[0]
-          params.ylabel ||= headers[1]
-        end
-        x = series[0].map(&:to_f)
-        y = series[1].map(&:to_f)
-        UnicodePlot.lineplot(x, y, **params.to_hc)
-      end
-    end
-
-    def get_method2(method1)
-      "#{method1}!".to_sym
-    end
-
-    def plot_xyy(data, method1, params)
-      headers = data.headers
-      series = data.series
-      method2 = get_method2(method1)
-      series.map! { |s| s.map(&:to_f) }
-      if headers
-        params.name   ||= headers[1]
-        params.xlabel ||= headers[0]
-      end
-      params.ylim ||= series[1..-1].flatten.minmax # why need?
-      plot = UnicodePlot.public_send(method1, series[0], series[1], **params.to_hc)
-      2.upto(series.size - 1) do |i|
-        UnicodePlot.public_send(method2, plot, series[0], series[i], name: headers&.[](i))
-      end
-      plot
-    end
-
-    def plot_xyxy(data, method1, params)
-      headers = data.headers
-      series = data.series
-      method2 = get_method2(method1)
-      series.map! { |s| s.map(&:to_f) }
-      series = series.each_slice(2).to_a
-      params.name ||= headers[0] if headers
-      params.xlim = series.map(&:first).flatten.minmax # why need?
-      params.ylim = series.map(&:last).flatten.minmax  # why need?
-      x1, y1 = series.shift
-      plot = UnicodePlot.public_send(method1, x1, y1, **params.to_hc)
-      series.each_with_index do |(xi, yi), i|
-        UnicodePlot.public_send(method2, plot, xi, yi, name: headers&.[]((i + 1) * 2))
-      end
-      plot
-    end
-
-    def plot_fmt(data, fmt, method1, params)
-      case fmt
-      when 'xyy'
-        plot_xyy(data, method1, params)
-      when 'xyxy'
-        plot_xyxy(data, method1, params)
-      else
-        raise "Unknown format: #{fmt}"
-      end
-    end
-
-    def lines(data, params, fmt = 'xyy')
-      check_series_size(data, fmt)
-      plot_fmt(data, fmt, :lineplot, params)
-    end
-
-    def scatter(data, params, fmt = 'xyy')
-      check_series_size(data, fmt)
-      plot_fmt(data, fmt, :scatterplot, params)
-    end
-
-    def density(data, params, fmt = 'xyy')
-      check_series_size(data, fmt)
-      plot_fmt(data, fmt, :densityplot, params)
-    end
-
-    def boxplot(data, params)
-      headers = data.headers
-      series = data.series
-      headers ||= (1..series.size).map(&:to_s)
-      series.map! { |s| s.map(&:to_f) }
-      UnicodePlot.boxplot(headers, series, **params.to_hc)
-    end
-
-    def colors(color_names = false)
-      UnicodePlot::StyledPrinter::TEXT_COLORS.each do |k, v|
-        print v
-        print k
-        unless color_names
-          print "\t"
-          print '  ●'
-        end
-        print "\033[0m"
-        print "\t"
-      end
-      puts
-    end
-
-    def check_series_size(data, fmt)
-      series = data.series
-      if series.size == 1
-        warn 'youplot: There is only one series of input data. Please check the delimiter.'
-        warn ''
-        warn "  Headers: \e[35m#{data.headers.inspect}\e[0m"
-        warn "  The first item is: \e[35m\"#{series[0][0]}\"\e[0m"
-        warn "  The last item is : \e[35m\"#{series[0][-1]}\"\e[0m"
-        exit 1
-      end
-      if fmt == 'xyxy' && series.size.odd?
-        warn 'YouPlot: In the xyxy format, the number of series must be even.'
-        warn ''
-        warn "  Number of series: \e[35m#{series.size}\e[0m"
-        warn "  Headers: \e[35m#{data.headers.inspect}\e[0m"
-        exit 1
-      end
-    end
-  end
-end