Speed is measured out of shares - it is all about luck. I suggest you to use cgminer API speed readings.
djeZo, could you display separate graphs? One for hashpower (as it is already) + separate graph showing accepted/rejected/invalid shares?
I don't want to do that - customers may judge rigs out of shares and we all know that share stats can greatly depend on how good pool is. Besides, share stats are reset the same moment you turn off cgminer or you turn off proxy, so there is not much point in doing this.