Forecasting on netapp with graphite source

I want to create forecasting for the Netapp storage on the Grafana . The current setup is like all the NetApp metrics are stored on the graphite. grafana fetches the data from graphite.

I want to calculate how many days does it take for 100% full of a disk.

There are three types of disk on Netapp Aggregates, Volumes, LUN

For Aggregate I having the metrics like size_avaliable, Size_used, Size_total, Size_used_perday.

I have used default graphite function like holtWintersConfidenceBands it is not working as expected.

Can anyone suggest me on achieving forecasting on grafana.