allow plotting to work without DISPLAY if saving to file
allow plotting to work without DISPLAY if saving to file

percyliang/fig,percyliang/fig,ppasupat/fig,ppasupat/fig,percyliang/fig,percyliang/fig,ppasupat/fig,percyliang/fig,ppasupat/fig,ppasupat/fig,ppasupat/fig,percyliang/fig