I’m here because I’m looking for an app that can periodically check my available bandwidth (not my USED bandwidth) and report the result over time in graphical form. I can’t tell from the feature list whether this is an available function or not.
I clearly don’t need the full functionality of GlassWire, and I already have a firewall. Is GlassWire something I should try for this, or can anyone suggest a different tool if that’s not the case?
Rudy
Try help@glasswire.com They can tell you. 
GlassWire is more focused on usage, than available.
In the past I have used a utility exactly as you described when I was having ISP problems. It would log connections which could then be graphed. It may have been DU Meter, but I’m not certain. You may find your router can provide the data.
The forum software confused me by displaying my reply to you twice. I deleted the extra copy, but that deleted both. 
Anyway, the comment was just a thank-you. I’ve sent them a message.