gnuplot5-x11 software package provides virtual package provided by gnuplot-x11, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install gnuplot5-x11
gnuplot5-x11 is installed in your system.
Make ensure the gnuplot5-x11 package were installed using the commands given below,
$ sudo dpkg-query -l | grep gnuplot5-x11*
You will get with gnuplot5-x11 package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Virtual packages |
| Package | gnuplot5-x11 |
| Version | |
| Description | virtual package provided by gnuplot-x11 |
| Sub Section | Virtual |