Pipe output directly to str and avoid additional memory allocations

Currently, there are lots of examples of code like this,
```
str << ('<tag ' << foo << ' ' << bar << '/>')
```
which create the string for the tag in memory before piping to str.

We can avoid creating all of these intermediate strings by dropping
the paranthesis and piping directly to str.

This relies on the `str` passed around to handle lots of small
appends. This is a problem when using RubyZip, but that is solved
in the next commit.

1 files changed, 4 insertions, 4 deletions

diff --git a/lib/axlsx/drawing/line_chart.rb b/lib/axlsx/drawing/line_chart.rb
index ab2012c7..21327820 100644
--- a/lib/axlsx/drawing/line_chart.rb
+++ b/lib/axlsx/drawing/line_chart.rb
@@ -76,14 +76,14 @@ module Axlsx
     # @return [String]
     def to_xml_string(str = +'')
       super(str) do
-        str << (+"<c:" << node_name << ">")
-        str << (+'<c:grouping val="' << grouping.to_s << '"/>')
-        str << (+'<c:varyColors val="' << vary_colors.to_s << '"/>')
+        str << "<c:" << node_name << ">"
+        str << '<c:grouping val="' << grouping.to_s << '"/>'
+        str << '<c:varyColors val="' << vary_colors.to_s << '"/>'
         @series.each { |ser| ser.to_xml_string(str) }
         @d_lbls.to_xml_string(str) if @d_lbls
         yield if block_given?
         axes.to_xml_string(str, :ids => true)
-        str << (+"</c:" << node_name << ">")
+        str << "</c:" << node_name << ">"
         axes.to_xml_string(str)
       end
     end