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If you want to select a sub-set of agents, then use the global Agent filter to first select the sub-set of agents. Hold down the Ctrl key to select multiple agents.
The sub-set of agents will now appear in the left hand Agent Filter where you can now select which agent to compare to the subset of agents.
You can use the this feature to compare one agent against another agent (by only selecting two agents in the global Agent filter).
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