Use warn.

Rakefile

@@ -4,8 +4,8 @@ require 'bundler'
 begin
   Bundler.setup(:development, :doc)
 rescue Bundler::BundlerError => e
-  STDERR.puts e.message
-  STDERR.puts "Run `bundle install` to install missing gems"
+  warn e.message
+  warn "Run `bundle install` to install missing gems"
   exit e.status_code
 end
 

lib/dm-visualizer/graphviz.rb

@@ -5,7 +5,7 @@
 rescue Gem::LoadError => e
   raise(e)
 rescue ::LoadError
-  STDERR.puts "GraphViz not available. Install it with: gem install ruby-graphviz"
+  warn "GraphViz not available. Install it with: gem install ruby-graphviz"
 end
 
 module DataMapper

lib/dm-visualizer/project.rb

@@ -307,7 +307,7 @@ def each_relationship
       #   The message to print.
       #
       def log(message)
-        STDERR.puts "dm-visualizer: #{message}"
+        warn "dm-visualizer: #{message}"
       end
 
       #