Scoobypedia discusses the Subaru knock mitigation algorithm in excruciating detail: http://scoobypedia.co.uk/index.php/Knowledge/SubaruKnockControlStrategyExplained
The implication of your chart (very helpful apart from the axis autoscaling being useless sometimes) is that there are 4 knock events during your drive, possibly corresponding the some abrupt changes in throttle, although without particular pattern. The knock strategy does what it does around these events, but the response is hard to predict due to the complexity of the response.
I can't say whether knock events are common or uncommon having not been able to log this myself.