When I have an existing, working Graphana setup and I add a whole new type of device, is there some way I need to update Graphana to have it grab the various states types that did not exist when I installed Graphana?
Sometimes, yes. There's two ways to add to the states/properties that are included: by including the state/property (across all devices), or by adding a specific device (and therefore, all states/properties are included for that single device). This is found in the Plugin -> Configure -> Inclusion / Exclusion Criteria. If you add a state/property, and add a new device with the same state/property, then it will automatically begin to log. This is why I include common Indigo states in the defaults for the configuration.
I've recently installed solar, and I can't find the "watts" sensorValue.
As you found, use the explore device as a guide. Go into Plugin -> Configure -> Inclusion / Exclusion Criteria and add "state.watts" to the included states. This will begin to log that state to Influx, which will make it available to Grafana.
These visualizations are built into Grafana. The SingleStat panel has been renamed to "Stat" in the latest versions of Grafana, which shows a single statistic and a background line chart of the history. But, the plugin is behind on the grafana version. I suggest looking at the grafana documentation, it's quite good.
If you are technically inclined, I suggest using my Grafana plugin for the logging, and InfluxDB, but to install Grafana outside of my plugin (it can simply be turned off in the plugin config). This way you'll get the latest and greatest of Grafana, versus relying on my plugin updates to maintain.