force add double quotes to string data

[kojix2]

issue

require 'numo/gnuplot'

x = ['a','b','c','d','e']
y = [1,2,3,4,5]

Numo.gnuplot do
  set boxwidth:0.5
  set title:"plot with boxes"
  set style:[fill:'solid']
  plot x,y, using:[2,'xtic(1)'], with:'boxes'
end

Error

Numo::Gnuplot::GnuplotError: 

gnuplot> e
         ^
         line x: invalid command

a 1 b 2 c 3 d 4 e 5 <- Gnuplot think it's end-of-input e

(eから始まる文字列を含めるとエラーが出るようです。よろしくおねがいします。）