'plotcount.py isles.dat show' doesn't open a window

[Peter Steinbach]

As mentioned here, the call to

$ python3 plotcount.py isles.dat show

should open a window to produce a plot. On my system:

Python 3.7.5 (default, Oct 17 2019, 12:09:47) 
[GCC 9.2.1 20190827 (Red Hat 9.2.1-1)] on linux

Nothing happens.

[Peter Steinbach]

$ python3 plotcount.py isles.dat isles.png and $ python3 plotcount.py isles.dat ascii work as expected.