Revert "Set the Gem Ruby version to 2.6 or higher"

I changed my mind, it better work on 2.4!

This reverts commit 2649959.

Author: kojix2

lib/youplot/backends/processing.rb

@@ -21,7 +21,7 @@ def count_values(arr, tally: true, reverse: false)
           # compare values
           r = b[1] <=> a[1]
           # If the values are the same, compare by name
-          r = a[0] <=> b[0] if r.zero?
+          r = a[0] <=> b[0] if r == 0
           r
         end
 

lib/youplot/parser.rb

@@ -164,8 +164,8 @@ def sub_parser_add_xscale
     end
 
     def sub_parser_add_canvas
-      canvas_types = UnicodePlot::Canvas::CANVAS_CLASS_MAP.keys.join(', ')
-      sub_parser.on_head('--canvas STR', String, 'type of canvas', "(#{canvas_types})") do |v|
+      canvas_types = UnicodePlot::Canvas::CANVAS_CLASS_MAP.keys.join(", ")
+      sub_parser.on_head('--canvas STR', String, "type of canvas", "(#{canvas_types})") do |v|
         params.canvas = v.to_sym
       end
     end

test/youplot/iris_test.rb

@@ -109,7 +109,7 @@ def fixture(fname)
     assert_equal fixture('iris-boxplot.txt'), @stderr_file.read
   end
 
-  # Yeah, lineplot/lineplots don't make sense too.
+  # Yeah, lineplot/lineplots don't make sense too. 
   # Just checking the behavior.
 
   test :c do

youplot.gemspec

@@ -12,7 +12,7 @@ Gem::Specification.new do |spec|
   spec.description   = 'A command line tool for Unicode Plotting'
   spec.homepage      = 'https://github.com/kojix2/youplot'
   spec.license       = 'MIT'
-  spec.required_ruby_version = Gem::Requirement.new('>= 2.6.0')
+  spec.required_ruby_version = Gem::Requirement.new('>= 2.4.0')
 
   spec.files         = Dir['*.{md,txt}', '{lib,exe}/**/*']
   spec.bindir        = 'exe'