I’d recommend just getting a 230V to 9V Euro wall transformer and replacing your 120V transformer with that. It has a standard Euro two pin plug. That’s what I use on my own home IoTaWatt. I adapted the plug by installing a standard us 240V socket, and changed the plug on a simple Euro small power strip.
Calibrate the VT with a voltmeter as detailed in the docs, then divide the resulting calibration by 2. You will now have a 120V voltage reference that is exactly half of your 240V supply. Create an output that doubles it and you can plot your 240V supply.
IMO this is a superior voltage reference as explained here 120V/240V split phase voltage reference