This finally solves the how-do-I-make-space-for-shell-prompts problem:
- plot.py -H0 => use full terminal height
- plot.py -H-1 => use height-1, making space for shell prompts
- plot.py -H => automatic based on other flags
While also allowing other carveouts in case your prompt takes up more
than 1 line.
Unfortunately this does make -H (no arg) subtly different from -H0, but
sometimes you can't have everything.
6c36f76ffd