Introduce simplecov

test/test_helper.rb

@@ -1,4 +1,6 @@
-$LOAD_PATH.unshift File.expand_path('../lib', __dir__)
+require 'simplecov'
+SimpleCov.start
+
 require 'uplot'
 
 require 'test/unit'

uplot.gemspec

@@ -24,5 +24,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'bundler'
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'rubocop'
+  spec.add_development_dependency 'simplecov'
   spec.add_development_dependency 'test-unit'
 end