Many thanks for you suggestions. You gave me a couple of things to look at further.
Am able to get the start time field from log entry
And can get the duration in seconds also from log entry.
Can convert the start time to epoch time and add on duration
to get the end time (as an epoch value or as a regular time stamp format).
But your suggestion "timechart sum(transmit_per_second)" would chart
the "transmit_per_second" against "_time" (the time stamp splunk gets the log at) , as opposed to the time in the log entry.
And gets the sum for this particular time rather than going showing a bar
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