[plotterparameter] Fix writing

The write method was wrong as well as there were missing
pieces in the class for it to work properly.
2 jobs for 78_fix_plotterparameter in 15 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #164372
docker-build
build_linux_36

00:14:26

65.0%
passed #164373
macosx
build_macosx_36

00:15:57

65.0%