show the help option on usage

lib/uplot/command/parser.rb

@@ -81,6 +81,12 @@ module Uplot
           opt.on('--fmt VAL', String, 'xyxy : header is like x1, y1, x2, y2, x3, y3...', 'xyy  : header is like x, y1, y2, y2, y3...') do |v|
             @fmt = v
           end
+          # Optparse adds the help option, but it doesn't show up in usage.
+          # This is why you need the code below.
+          opt.on('--help', 'print sub-command help menu') do
+            puts opt.help
+            exit
+          end
           opt.on('--debug', TrueClass, 'print preprocessed data') do |v|
             @debug = v
           end