diff --git a/lib/youplot/command.rb b/lib/youplot/command.rb
index 51dab9e..d91c178 100644
--- a/lib/youplot/command.rb
+++ b/lib/youplot/command.rb
@@ -169,7 +169,7 @@ module YouPlot
     def output_data(input)
       # Pass the input to subsequent pipelines
       case options[:pass]
-      when IO
+      when IO, StringIO
         options[:pass].print(input)
       else
         if options[:pass]