Forcastgraphs are only available inside Enterprise edition. Why not starting with a normal graph like in the KB?
lq "GET services\nFilter: host_name = localhost2\nFilter: service_description = Filesystem /\nColumns: host_name\nColumns: service_description\nColumns: rrddata:m1:fs_used.max,1024,/:1614839543:1614929543:30\nOutputFormat: python"
Here you must pay attention as the timestamp must be calculated by your script. The two are start and end of the requested timeperiod.