Ambari provides some inbuilt alerts to findout the Weekly/Daily growth in the HDFS usage.
Ambari UI --> Alerts (Tab) --> "Alert Definition Filter" Search for "HDFS Storage Capacity Usage"
This service-level alert is triggered if the increase in storage capacity usage deviation has grown beyond the specified threshold within a given period. This alert will monitor Daily and Weekly periods.
Please see: https://docs.hortonworks.com/HDPDocuments/Ambari-184.108.40.206/bk_ambari-user-guide/content/hdfs_service_a...
However if you want to get this data for 6 months then you might have to write your own custom alert script.
\Sometime back i have written a basic example of how we can have our own custom ambari alert.
Grafana basically fetches data from AMS (Ambari Metrics Collector) using APIs so the data need to be available on AMS first.