Add tally method for older rubies

lib/uplot/command.rb

@@ -205,3 +205,14 @@ module Uplot
     end
   end
 end
+
+# backports
+# https://github.com/marcandre/backports/blob/master/lib/backports/2.7.0/enumerable/tally.rb
+unless Enumerable.method_defined? :tally
+  module Enumerable
+    def tally
+      # NB: By spec, tally should return default-less hash
+      each_with_object(Hash.new(0)) { |item, res| res[item] += 1 }.tap { |h| h.default = nil }
+    end
+  end
+end