You must enable Voltage under Automatic Uploads. This will plot the DC or one phase of your power Voltage. Select DC to see the inverter’s DC voltage of your inverter or V1 to see the phase 1 AC power voltage. If you want to see both you will have to be a Donator and make them visible on Extended Data page and map the desired values to that page. Being a Donator gives you the ability to upload and set of variables from your system to the Extended variables page. You will need to upload them by some means, usually an API, or your own application through the PVOutput API. Most private weather station devices allow you to connect to the PVOutput API and automaticlally upload its data to PVOutput. It takes a little effort on your part and there is a lot of help available at PVOutput to get it working.