1 files changed, 8 insertions, 6 deletions
diff --git a/lib/axlsx/drawing/num_data_source.rb b/lib/axlsx/drawing/num_data_source.rb
index 46f6ba71..af5e31ab 100644
--- a/lib/axlsx/drawing/num_data_source.rb
+++ b/lib/axlsx/drawing/num_data_source.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Axlsx
   # A numeric data source for use by charts.
   class NumDataSource
@@ -42,17 +44,17 @@ module Axlsx
 
     # serialize the object
     # @param [String] str
-    def to_xml_string(str = "")
-      str << ('<c:' << tag_name.to_s << '>')
+    def to_xml_string(str = +'')
+      str << (+'<c:' << tag_name.to_s << '>')
       if @f
-        str << ('<c:' << @ref_tag_name.to_s << '>')
-        str << ('<c:f>' << @f.to_s << '</c:f>')
+        str << (+'<c:' << @ref_tag_name.to_s << '>')
+        str << (+'<c:f>' << @f.to_s << '</c:f>')
       end
       @data.to_xml_string str
       if @f
-        str << ('</c:' << @ref_tag_name.to_s << '>')
+        str << (+'</c:' << @ref_tag_name.to_s << '>')
       end
-      str << ('</c:' << tag_name.to_s << '>')
+      str << (+'</c:' << tag_name.to_s << '>')
     end
   end
 end