If you don't specify a bucket option (like span, minspan, bins) while running the timechart, it automatically does further bucket automatically, based on number of result. By Specifying minspan=10m, we're ensuring the bucketing stays the same from previous command. You can use span instead of minspan there as well.
In your timechart command, you're using by clause and what it does is it create a column (along with column _time) for each of the value of the field in by clause (Web.action here). Also, by default it shows only 10 columns and groups remaining as 11th field called OTHERS. The option useother=false will remove that OTHER column.
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