Howto monitor VLAN traffic from a HP switch using cacti?

[cc]

hi

I have cacti Version: 0.8.8f_1 installed on FreeBSD 10.0-RELEASE-p12 and I would like to monitore VLAN traffic (bits per second) from a HP 5400 zl Switch.
I have configured, but the graph is empty.
BTW to monitore any other interface without VLAN from the same switch works well.
Knows anyone a solution howto monitor VLAN using cacti?