diff --git a/Gemfile b/Gemfile
index 43abf00..90a7c73 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,6 +5,13 @@ source 'https://rubygems.org'
 # Specify your gem's dependencies in youplot.gemspec
 gemspec
 
+group :development do
+  if RUBY_VERSION >= '3.0'
+    gem 'steep', require: false
+    gem 'typeprof'
+  end
+end
+
 group :test do
   gem 'rake'
   gem 'simplecov'
diff --git a/Steepfile b/Steepfile
new file mode 100644
index 0000000..edee931
--- /dev/null
+++ b/Steepfile
@@ -0,0 +1,8 @@
+D = Steep::Diagnostic
+
+target :lib do
+  signature "sig"
+  check "lib" # Directory name to check
+
+  configure_code_diagnostics(D::Ruby.silent) # `silent` diagnostics setting
+end
diff --git a/sig/youplot/backends/processing.rbs b/sig/youplot/backends/processing.rbs
new file mode 100644
index 0000000..9830361
--- /dev/null
+++ b/sig/youplot/backends/processing.rbs
@@ -0,0 +1,7 @@
+module YouPlot
+  module Backends
+    module Processing
+      def count_values: ([], ?tally: true, ?reverse: false) -> Array[Array[untyped]]
+    end
+  end
+end