K7ITM wrote:

It would be good if they plotted the range on something like a Smith
chart. It should be done for different frequencies; it's rare indeed
for a tuner to be able to match the same range of impedances over its
frequency range. Some tuner topologies even have "holes" in their
coverage.

Exactly ! You will find here an example:
http://www.darc.de/distrikte/c/09/Claude/Z-Match.pdf

73 de Claude