Most of the time, monitoring usable free size of oracle tablespaces is helpful. Especially for production systems. Keeping that statistical data for some time is also meaningful so as to see how much new data enter into the database.
With the following bash script each tablespace's free size can be sent to graphite database. Just query oracle data dictionary views (dba_tablespaces, dba_data_files, dba_free_space) then send each value to graphite using netcat.
On the side of graphite carbon storage-schemas.conf whisper file schema definitions must be updated like the following example. This file is scanned for changes every 60 seconds, so no need to reload any service.
pattern = ^ora_tbls.*.free_space_mb$
retentions = 10m:90d
I am using grafana to visualize the metrics. It looks like: