www.neuron.yale.edu

As I will be running NEURON on a supercomputer, I'd like to make plots off from saved text data.
Anyone had some experience in a larger computing environment?

"How do I collect data . . . "

Use the Vector class's record() method. Documented in the Programmer's Reference.

As I will be running NEURON on a supercomputer . . .

Make friends with your local tech support people.

Work through the "Scripting NEURON with Python" tutorial (see link on the Documentation page of NEURON's web site https://neuron.yale.edu/neuron/docs). In particular, part 4 of the "ball and stick tutorials" contains useful information, especially with regard to dealing with simulation output that resides on multiple hosts.

Is there a doc for the hoc based programmers?

Indeed there is. It's the paper cited under the heading
Parallel communication in NEURON
in the 4th "ball and stick" tutorial--
Hines M.L. and Carnevale N.T.
Translating network models to parallel hardware in NEURON,
Journal of Neuroscience Methods 169 (2008) 425–455

The citation in the tutorial has a link to a preprint of that article.

There's also a HOC version of the programmer's reference at: https://www.neuron.yale.edu/neuron/stat ... index.html