Sorry I was not able to reply earlier.
You should be able to generate the graph using two options:
1. dfm graph -s 18000 -e 0 -h 500 -w 1000 -F gif volume-usage <controller_name>:<volume_name> > test.gif
2. Obtain the volume dfm object id and run the following:
dfm graph -s 18000 -e 0 -h 500 -w 1000 -F gif volume-usage <object id>
Do let me know how this goes.