MPL plotting: reinstate dry-run option

@max.knobbe pointed out that --dry-run was broken and looking a bit closer, I also don't see --write-files being used anywhere.

• For the former: I think we do still want to write out the py files, but we just don't want to execute them, no?
• For the latter: I've removed this for now 🤷

Also noticed that the --single option was broken - reinstated that now as well.