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