gnuplot warning: "plotting with an unknown terminal" I'm trying to use gnuplot on my MacBook Air (Mac OS X 10.11.3).
I have installed XQuartz by using homebrew after I installed gnuplot (with this YouTube video: https://www.youtube.com/watch?v=AR-GUSkqMCU).
But gnuplot did not work and terminal showed: Terminal type is now 'unknown'. 

For example, I wanted to plot sin(x) and I got a error message as the following:

gnuplot&gt; plot(sin(x))
WARNING: Plotting with an 'unknown' terminal.
No output will be generated. Please select a terminal with 'set terminal'.
You need to install DirectX11 and XQuartz.
Restart your Mac and Retry.

https://csml-wiki.northwestern.edu/index.php/Binary_versions_of_Gnuplot_for_OS_X:


  Binary versions of Gnuplot for OS X/macOS. These are primarily
  intended for users who normally work on Linux and wish to replicate
  their tools on a Macintosh.
  
  
  Compiled on OS X 10.11.6 (El Capitan). Confirmed to work also on macOS 10.13 (High Sierra).
  Prior to installing this, you need to install X11/XQuartz (http://xquartz.macosforge.org/landing/).
  The versions provided here include the GNU Readline library.
  In addition to the standard terminals, the PDF terminal is included as well.
  Simply download the package from one of the links below and install. You can then invoke gnuplot from a terminal window via
  /usr/local/bin/gnuplot
  Currently available versions:
  
  
  5.2.2
  5.2.3