More specifically, why should the hourly average visit duration graph look like this:
It's as if there's some sort of daily artefact in the data or method of calculation. This pattern doesn't appear in any other graphs.
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Sign up to join this communityhttps://support.google.com/analytics/answer/1006253?hl=en explains how they compute the average visit duration: in short the formula is (The time of the first hit on the last page - the first hit on the first page) / (number of visited pages)
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Regarding your second question pertaining to your graph, try https://stackoverflow.com/q/15841370/395857