Add `imgname` parameter to be able to save a plot to a file instead of
showing it. Use-case: platforms without GUI like docker containers.
Add `labels` parameter to histogram plot. Use-case: replacing long
commands with human friendly labels.
Before this patch, executing directly one of these scripts, for example
`./plot_hystogram.py` in a unix-like environment meant that the default
system-level python would be used, regardless of an eventual activated
virtualenv.
This was due to the "#!/usr/bin/python" shebang.
Changing it to "/usr/bin/env python" is a fairly standard practice, keeps intact
the compatiblity with the system level python, and allows a user to run in a
virtualenv if he wants.
