Calculate delta time between two packets from Lua dissector and display on a graph

Calculating the time delta in a Wireshark Lua script is not hard, but there is no way to have the results be graphed by Wireshark, other than to export the data to a file like you are now. Supporting such a thing has been on my to-do list (for the Qt version of Wireshark, not GTK, fwiw).

To calculate the delta time, you'd use a Lua table. The table's keys would be your protocol's reference numbers, and the values would be the time (use Lua's built-in os.time() or os.clock()). So when you get a request packet, you insert a new entry in the table of the ref number as the key and the current time as the value, and when you get a response you look up the ref number in the table, and subtract the entry's time value from current time. (actually you'll want to do the lookup first on the request too, in case the request is retransmitted, since you want to use the originally transmitted request time presumably)

Also remember to clear the table before each run of the capture, so that it won't grow forever as people open/close capture files or start/restart live capturing of your protocol. The easiest way to do this is to clear the table (i.e., reset the variable to a new table) in the myproto.init() function for your new protocol. The init() function is automatically called by Wireshark, similar to the dissector() one, but it's called each time a new capture file is opened or a live capture is started.