What is the unit of tcp.time_delta in IO Graph

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I created an IO graph and set AVG(tcp.time_delta) as the Y axis. What I expected is to show average time_delta in seconds in Y axis. But what I actually got is values such as 210^6 and 1.210^7 (see the image below).

Is it something with the unit of tcp.time_delta? Is the tcp.time_delta in second, millisecond or nanosecond by default? And how can I achieve my expected behavior to show Y axis in second?

Thanks a alot.