valid values Y Field values IO Graphs

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I have been using wireshark for a very long time, but...Im now trying to do something that I have never done. I want to chart network traffic using the I/O Graphs capability. I /hope/ to be able to chart the Y Axis, using COUNT FIELDS(Y Field), but I dont know what to type into the Y Field to subselect into the packet-header and/or data offset. Been googling around fro hrs....trying to find answers, even man pages are silent, as if wireshark main site. Any pointers from this community would be helpful. Thanks in advance.