How can I resolve a permission error when installing gnuplot from homebrew? I am trying to install gnuplot on my mac. after trying    brew install gnuplot, I get the following errors:    


  You must 'brew link lua jpeg' before gnuplot can be installed


And when I try    brew link lua jpeg I get the following error:    


  Could not symlink file: /usr/local/Cellar/lua/5.1.4/include/lualib.h
  /usr/local/include is not writable. You should change its permissions.  


Any suggestion? BTW, my XCode and brew are updated.
sudo chown $(whoami) /usr/local/include
chmod +w /usr/local/include
brew link lua