Add perf graph for freeipmi fan speed

we are using freeipmi for some servers. The service view shows
|IPMI Sensor Fan_System_Fan_1|||Status: OK, 2380.00 RPM|
|IPMI Sensor Temperature_Baseboard_Temp|||Status: OK, 30.00 C|

but only the temperature has performance data and the history graph.
I’d like to see also the history of the fan speed.