How to view the packet diagram

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The packet diagram is one of the 4 options that may be selected for display in the 3 panes of the UI.

To display the packet diagram open the Edit -> Preferences -> Appearance -> Layout dialog and using the radio buttons select the packet diagram option for one of the panes, it's usual to select it in pane 3 instead of the packet bytes display as the packet list (pane 1) and packet details (pane 2) are essential to the use of Wireshark.