user@myhost:~# python3 --version
Python 3.6.9
user@myhost:~# python3 bin/zpool-iostat-viz
CRASH! Sorry!
Please report this error at 
https://github.com/chadmiller/zpool-iostat-viz/issues/new

Paste the following:
Traceback (most recent call last):
  File "bin/zpool-iostat-viz", line 303, in <module>
    raise exc
  File "bin/zpool-iostat-viz", line 292, in <module>
    curses.wrapper(lambda window: main(window, parsed_args.pools, parsed_args.file))
  File "/usr/lib/python3.6/curses/__init__.py", line 94, in wrapper
    return func(stdscr, *args, **kwds)
  File "bin/zpool-iostat-viz", line 292, in <lambda>
    curses.wrapper(lambda window: main(window, parsed_args.pools, parsed_args.file))
  File "bin/zpool-iostat-viz", line 280, in main
    display_measurements_for_device(pool, filename, window)
  File "bin/zpool-iostat-viz", line 152, in display_measurements_for_device
    stats = get_stats(pool, filename)
  File "bin/zpool-iostat-viz", line 63, in get_stats
    zpool_cmd = subprocess.run(["zpool", "iostat", "-wvHp"] + (pool_names if pool_names else []), check=True, capture_output=True, encoding="UTF-8")
  File "/usr/lib/python3.6/subprocess.py", line 423, in run
    with Popen(*popenargs, **kwargs) as process:
TypeError: __init__() got an unexpected keyword argument 'capture_output'

I have created a "port" of zpool-iostat-viz in FreeBSD that contains a few changes relative to your version:

The following functionality has been added:

--digits option: while letters give high precision output, it is not always easy to remember their order, especially with quickly changing displays. Digits give less resolution, but are easier to interpret.

--symbols option: even less steps, but even more intuitive. Symbols have been chosen to cover more of the character cell for higher values. This mode could be combined with a blank white color palette, since colors tend to reduce the effect of the non-black fraction of the character cell.

Two interactive commands have been added:

'd': switch --diff mode on or off 'm': toggle between per measurement or per device mode

I have once got a run-time error when toggling modes, but could not reproduce it in later attempts. AFAIU the program, the state between updates should be stored in a way that allows to toggle modes at any time (between updates).

The wording of the man-page would probably benefit from a review by a native English speaker (which I'm obviously not).

probably you need some more info, besides this? (system is debian 9 (stretch) updated to latest available)

Traceback (most recent call last): File "./zpool-iostat-viz", line 290, in raise exc File "./zpool-iostat-viz", line 279, in curses.wrapper(lambda window: main(window, parsed_args["diff"], parsed_args["parts"], parsed_args["file"], parsed_args["by"] or "m")) File "/usr/lib/python3.5/curses/init.py", line 94, in wrapper return func(stdscr, *args, **kwds) File "./zpool-iostat-viz", line 279, in curses.wrapper(lambda window: main(window, parsed_args["diff"], parsed_args["parts"], parsed_args["file"], parsed_args["by"] or "m")) File "./zpool-iostat-viz", line 241, in main render_stats(window, transformation_function, should_show_differential, pool, filename) File "./zpool-iostat-viz", line 132, in render_stats stats = transform(get_stats(pool, filename)) File "./zpool-iostat-viz", line 65, in get_stats zpool_cmd = subprocess.run(["zpool", "iostat", "-wvHp", "--"] + (pool_names if pool_names else []), check=True, stdout=subprocess.PIPE, encoding="UTF-8") File "/usr/lib/python3.5/subprocess.py", line 383, in run with Popen(*popenargs, **kwargs) as process: TypeError: init() got an unexpected keyword argument 'encoding'

Fix program crash if the "m" command is used and "current" is higher than supported by the current stats array length.

The "m" command has been part of a previous pull request that has been merged. I forgot that the stats array length can change and that the stats_history contents depends on the transform function selected.

This patch fixes this issue. The value of "current" is reset to one that is valid for all possible stats array lengths, and stats_history is reset to an empty array to prevent incompatible data to be used in a difference calculation.