chore(dockerfile): Add numo-gnuplot

Author: kozo2

Dockerfile

@@ -2,7 +2,7 @@ FROM ruby
 
 USER root
 
-RUN apt-get update && apt-get install -y emacs25-nox vim-nox mlocate locales-all tmux && \
+RUN apt-get update && apt-get install -y emacs25-nox vim-nox mlocate locales-all tmux gnuplot && gem install numo-gnuplot\
     git config --global alias.co checkout && \
     git config --global alias.br branch && \
     git config --global alias.ci commit && \