looking for a bit of help in a forecast calculation

I pull the forecasts currently from Darksky into Homeassitant (house automation). All the values the HA has are fed into InfluxDB. As part of my HA instalation I use Node-Red for automations and calcs etc. However I also use Grafana to visualise some of the data.

I am pulling the past data out of the InfluxDB using Node-Red

I have attached a screenshot of a display I have set up in Grafana just as a quick comparison