delete graphical data after a period?
Data are stored in your Database (e.g. InfluxDB) and then Grafana has Time Range settings for displaying the graphs.
If you want to remove data older than x days, you could check the use of retention policy in InfluxDB or use queries to delete old data.
Ref: https://docs.influxdata.com/influxdb/v1.3/query_language/database_management